1. 1. DEVELOPED SPECIFICALLY FOR THE IB DIPLOMA CHRIS HAMPER HIGHER LEVEL (plus STANDARD LEVEL OPTIONS) P E A R S O N BACCALAUREATE A01_IBPH_SB_HIGGLB_4426_PRE.indd 1 29/6/10 11:46:58
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3. 3. iii Every effort has been made to contact copyright holders of material reproduced in this book. Any omissions will be rectified in subsequent printings if notice is given to the publishers. The assessment statements and various examination questions have been reproduced from IBO documents and past examination papers. Our thanks go to the International Baccalaureate Organization for permission to reproduce its intellectual copyright. This material has been developed independently by the publisher and the content is in no way connected with nor endorsed by the International Baccalaureate Organization. Websites There are links to relevant websites in this book. In order to ensure that the links are up-to-date, that the links work, and that the sites are not inadvertently linked to sites that could be considered offensive, we have made the links available on our website at www.heinemann.co.uk/hotlinks. When you access the site, the express code is 4426P. A01_IBPH_SB_HIGGLB_4426_PRE.indd 3 29/6/10 11:46:59
4. 4. iv Dedication There are several people I would like to dedicate this book to, firstly my father who persuaded me to take up teaching, my family who supported me through the writing process and all the students I have ever taught whose questions came back to me throughout the lonely hours of writing. There were also five students, Bante, Liuliang, Xiaolong, DANAMONA and Giovanni who deserve special thanks for their help in proofing the problem solutions. Chris Hamper A01_IBPH_SB_HIGGLB_4426_PRE.indd 4 29/6/10 11:46:59
5. 5. Contents Introduction vii 1: Physics and physical measurement 1 1.1Fundamental quantities 1 1.2Measurement 4 1.3Collecting data 6 1.4Presenting processed data 7 1.5Vectors and scalars 10 2: Mechanics 17 2.1Kinematics 17 2.2Free fall motion 22 2.3Graphical representation of motion 23 2.4Projectile motion 27 2.5Forces and dynamics 31 2.6Newtons laws of motion 35 2.7The relationship between force and acceleration 37 2.8Newtons third law 41 2.9Work, energy and power 47 2.10 Uniform circular motion 57 3: Thermal physics 66 3.1Thermal concepts 66 3.2 Thermal properties of matter 71 3.3Kinetic model of an ideal gas 78 3.4Thermodynamics 81 3.5 Thermodynamic processes 86 3.6The second law of thermodynamics 91 4: Simple harmonic motion and waves 100 4.1Kinematics of simple harmonic motion 100 4.2Energy changes during simple harmonic motion (SHM) 108 4.3Forced oscillations and resonance 110 4.4Wave characteristics 115 A01_IBPH_SB_HIGGLB_4426_PRE.indd 5 29/6/10 11:46:59
6. 6. Contents 4.5Wave properties 122 4.6Standing (stationary) waves 130 4.7The Doppler effect 135 4.8Diffraction at a single slit 138 4.9Resolution 141 4.10 Polarization 144 4.11 Uses of polarization 146 5: Electrical currents 157 5.1Electric potential difference, current and resistance 157 5.2Electric circuits 164 6: Fields and forces 181 6.1Gravitational force and field 181 6.2Gravitational potential 186 6.3Escape speed 190 6.4Orbital motion 192 6.5Electric force and field 195 6.6Electric potential 200 6.7Magnetic force and field 204 6.8Electromagnetic induction 209 6.9Alternating current 214 6.10 Transmission of electrical power 218 7: Atomic and nuclear physics 225 7.1Atomic structure 225 7.2The quantum nature of light 228 7.3The wave nature of matter 232 7.4Quantum models of the atom 236 7.5Nuclear structure 238 7.6Radioactive decay 244 7.7Half-life 251 7.8Nuclear reactions 256 8: Energy, power and climate change 265 8.1Energy degradation and power generation 265 8.2World energy sources 269 8.3Fossil fuel power production 273 8.4Nuclear power 277 A01_IBPH_SB_HIGGLB_4426_PRE.indd 6 29/6/10 11:46:59
7. 7. 8.5Solar power 285 8.6Hydroelectric power 287 8.7Wind power 290 8.8Wave power 292 8.9 The greenhouse effect 294 8.10 Global warming 301 8.11 What might happen and what can be done? 305 9: Digital technology 311 9.1Analogue and digital signals 311 9.2Data capture; digital imaging 323 10 (Option E): Astrophysics 332 10.1Introduction to the universe 332 10.2Stellar radiation and stellar types 338 10.3Light from stars 340 10.4Types of star 344 10.5Stellar distances 347 10.6Magnitude 351 10.7Stellar processes and stellar evolution 354 10.8Cosmology 358 10.9Development of the universe 362 11 (Option F): Communications 369 11.1Radio communication 369 11.2Digital signals 376 11.3Optic fibre transmission 385 11.4Channels of communication 390 11.5Electronics 396 11.6The mobile phone system 403 12 (Option G): Electromagnetic waves 411 12.1The nature of EM waves and light sources 411 12.2X-rays 421 12.3Two-source interference of waves 424 12.4Interference by thin films 428 12.5Interference by air wedges 431 12.6Diffraction grating 432 12.6X-ray diffraction 434 A01_IBPH_SB_HIGGLB_4426_PRE.indd 7 29/6/10 11:46:59
8. 8. viii Contents 12.8Lenses and image formation 437 12.9Optical instruments 442 12.10 Aberrations 449 13 (Option H): Relativity 456 13.1Special relativity 456 13.2Time dilation 463 13.3Length contraction 467 13.4Mass and energy 470 13.5General relativity 475 13.6Space-time and black holes 478 14 (Option I): Medical physics 485 14.1The ear and hearing 485 14.2Medical imaging 492 14.3Radiation in medicine 502 15 (Option J): Particles 512 15.1Description and classification of particles 512 15.2Fundamental interactions 515 15.3Particle accelerators 520 15.4 Particle detectors 526 15.5Quarks 532 15.6Feynmann diagrams 536 15.7Experimental evidence for the quark and standard models 539 15.8Cosmology and strings 543 16: Theory of knowledge 550 Internal assessment 560 Extended essay 572 Appendix: The eye and sight 577 Answers 582 Index 612 A01_IBPH_SB_HIGGLB_4426_PRE.indd 8 29/6/10 11:46:59
9. 9. ix Welcome to your new course! This book is designed to act as a comprehensive course book, covering the core and AHL material and all the options you might take while studying for the IB Diploma in Physics at Higher Level. It will also help you to prepare for your examinations in a thorough and methodical way. Content Chapters 1 to 9 provide coverage of the core and AHL material. Chapters 10 to 15 cover the options E to J. HL students are required to study two of these options. Within each chapter, there are numbered exercises for you to practise and apply the knowledge that you have gained. The exercises will also help you to assess your progress. Sometimes, there are worked examples that show you how to tackle a particularly tricky or awkward question. Worked example A car travelling at 30ms21 emits a sound of frequency 500Hz. Calculate the frequency of the sound measured by an observer in front of the car. Solution This is an example where the source is moving relative to the medium and the observer is stationary relative to the medium. To calculate the observed frequency, we use the equation f15 cf0 ______ (c 2 v) where f05 500Hz c5 330ms21 v5 30ms21 So f1 5 330 3 500_________ 330 2 30 5 550Hz At the end of each chapter, there are practice questions taken from past exam papers. Towards the end of the book, starting on page 582, you will find answers to all the exercises and practice questions. After the options chapters, you will find a Theory of Knowledge chapter, which should stimulate wider research and the consideration of moral and ethical issues in the field of physics. Following this, there are two short chapters offering advice on internal assessment and on writing extended essays. Finally, there is a short appendix which covers material on the eye and sight. This is not required study for HL students, but is part of Option A for SL students. Introduction A01_IBPH_SB_HIGGLB_4426_PRE.indd 9 29/6/10 11:46:59
10. 10. x Introduction Information boxes You will see a number of coloured boxes interspersed through each chapter. Each of these boxes provides different information and stimuli as follows. You will find a box like this at the start of each section in each chapter. They are the numbered objectives for the section you are about to read and they set out the content and aspects of learning covered in that section. In addition to the Theory of Knowledge chapter, there are TOK boxes throughout the book. These boxes are there to stimulate thought and consideration of any TOK issues as they arise and in context. Often they will just contain a question to stimulate your own thoughts and discussion. These boxes contain interesting information which will add to your wider knowledge but which does not fit within the main body of the text. Infinity We cant really take a mass from infinity and bring it to the point in question, but we can calculate how much work would be required if we did. Is it OK to calculate something we can never do? Antimatter It is also possible for a proton to change into a neutron and a positive electron plus a neutrino. The positron is in some way the opposite of an electron 2 it is called an antiparticle. The antineutrino _ nis also the antiparticle of the neutrino. Every particle has an antiparticle. Atoms made of negative positrons and positive electrons are called antimatter. Assessment statements 7.3.6 Draw and annotate a graph showing the variation with nucleon number of the binding energy per nucleon. 7.3.7 Solve problems involving mass defect and binding energy. 13.2.2 Describe how the masses of nuclei may be determined using a Bainbridge mass spectrometer. 13.2.1 Explain how the radii of nuclei may be estimated from charged particle scattering experiments. A01_IBPH_SB_HIGGLB_4426_PRE.indd 10 29/6/10 11:46:59
11. 11. xi These facts are drawn out of the main text and are highlighted. This makes them useful for quick reference and they also enable you to identify the core learning points within a section. These boxes indicate examples of internationalism within the area of study. The information in these boxes gives you the chance to think about how physics fits into the global landscape. They also cover environmental and political issues raised by your subject. These boxes can be found alongside questions, exercises and worked examples and they provide insight into how to answer a question in order to achieve the highest marks in an examination. They also identify common pitfalls when answering such questions and suggest approaches that examiners like to see. These boxes direct you to the Heinemann website, which in turn will take you to the relevant website(s). On the web pages you will find background information to support the topic, or video simulations and animations. Examiners hint: Since field strength g is a vector, the resultant field strength equals the vector sum. To view a simulation of random decay, visit www.heinemann.co.uk/hotlinks, enter the express code 4426P and click on Weblink 7.4. The electronvolt Remember 1eV is the KE gained by an electron accelerated through a p.d. of 1V. 1eV 5 1.6 3 10219 J Different cultures developed different systems for defining the different patterns in the stars. The Greeks had the 12 signs of the zodiac, the Chinese 28 xiu and the Indians 27 Nakshatra. A01_IBPH_SB_HIGGLB_4426_PRE.indd 11 29/6/10 11:46:59
12. 12. Using this book for SL Physics This book has been written for students studying HL physics but can also be used by SL students. If you are studying SL physics you do not have to study all of the HL core material. SL options SL options A, B and C are the same as the HL core: A Sight and Wave Phenomena This is the same as the AHL material in Chapter 4 plus the material on the eye in the Appendix (pp. 577581). B Quantum and Nuclear Physics This is the same as the AHL material in Chapter 7. C Digital Technology This is the same as the AHL material in Chapter 9 plus the section on electronics and the mobile phone in Chapter 11. So if you are an SL student and you study up to and including Chapter 9 you will have done the SL core plus three options (you only need to do two). The remaining SL options are parts of the HL options. D Relativity and Particles This is the first part of Chapter 13 and the first part of Chapter 15. E Astrophysics This is the first part of Chapter 10. F Communications This is the same as Chapter 11 without the part on electronics and the mobile phone. G Electromagnetic Waves This is the first part of Chapter 12. Worked Solutions Full worked solutions to all exercises and practice questions can be found online at www.pearsonbacc.com/solutions. Introduction xii A01_IBPH_SB_HIGGLB_4426_PRE.indd 12 29/6/10 11:46:59
13. 13. 1 1 Physics and physical measurement Position Physics is about modelling our universe. To do this, we need to define the things inside it. Each thing is different for many reasons, but one of the most important differences is their different positions. To define position, we use the quantity distance; this is how far the object is away from us. To quantify (put a number to) this difference we compare the distance with some standard measure (the metre rule). All distances can then be quoted as multiples of this fundamental unit, for example: The distance from Earth to the Sun 5 1.5 3 1011 m The size of a grain of sand 5 2 3 1024 m The distance to the nearest star 5 4 3 1016 m The radius of the Earth 5 6.378 3 106 m Standard form The quantities above are expressed in standard form. This means that there is only one number to the left of the decimal place. For example: 1600m in standard form is 1.6 3 103 m Fundamental quantities1.1 Assessment statements 1.1.1 State and compare quantities to the nearest order of magnitude. 1.1.2 State the ranges of magnitude of distances, masses and times that occur in the universe, from the smallest to the greatest. 1.1.3 State ratios of quantities as differences of orders of magnitude. 1.1.4 Estimate approximate values of everyday quantities to one or two significant figures and/or to the nearest order of magnitude. 1.2.1 State the fundamental units in the SI system. 1.2.2 Distinguish between fundamental and derived units and give examples of derived units. 1.2.3 Convert between different units of quantities. 1.2.4 State units in the accepted SI format. 1.2.5 State values in scientific notation and in multiples of units with appropriate prefixes. The metre The metre was originally defined in terms of several pieces of metal positioned around Paris. This wasnt very accurate so now one metre is defined as the distance travelled by light in a vacuum in 1 _______299792458of a second. It is also acceptable to use a prefix to denote powers of 10 Prefix Value T (tera) 1012 G (giga) 109 M (mega) 106 k (kilo) 103 c (centi) 1022 m (milli) 1023 (micro) 1026 n (nano) 1029 p (pico) 10212 f (femto) 10215 Exercise 1 Convert the following into metres (m) and write in standard form: (a) Distance from London to New York 5 5585km (b) Height of Einstein was 175cm (c) Thickness of a human hair 5 25.4m (d) Distance to edge of the universe 5 100000 million million millionkm M01_IBPH_SB_HIGGLB_4426_U01.indd 1 29/6/10 11:48:25
14. 14. 2 Physics and physical measurement1 The second The second was originally defined as a fraction of a day but todays definition isthe duration of 9192631770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium-133 atom. The kilogram The kilogram is the only fundamental quantity that is still based on an object kept in Paris. Moves are underway to change the definition to something that is more constant and better defined but does it really matter? Would anything change if the size of the Paris masschanged? Time When something happens we call it an event. To distinguish between different events we use time. The time between two events is measured by comparing to some fixed value, the second. Time is also a fundamental quantity. Some examples of times: Time between beats of a human heart 5 1s Time for the Earth to go around the Sun 5 1 year Time for the Moon to go around the Earth 5 1 month Mass If we pick different things up we find another difference. Some things are easy to lift up and others are difficult. This seems to be related to how much matter the objects consist of. To quantify this we define mass measured by comparing different objects to a piece of metal in Paris, the standard kilogram. Some examples of mass: Approximate mass of a man 5 75kg Mass of the Earth 5 5.97 3 1024 kg Mass of the Sun 5 1.98 3 1030 kg Volume The space taken up by an object is defined by the volume. Volume is measured in cubic metres (m3 ). Volume is not a fundamental unit since it can be split into smaller units (m 3 m 3 m). We call units like this derived units. Exercise 2 Convert the following times into seconds (s) and write in standard form: (a) 85 years, how long Newton lived (b) 2.5ms, the time taken for a mosquitos wing to go up and down (c) 4 days, the time it took to travel to the Moon (d) 2 hours 52min 59s, the time for Concord to fly from London to New York Exercise 3 Convert the following masses to kilograms (kg) and write in standard form: (a) The mass of an apple 5 200g (b) The mass of a grain of sand 5 0.00001g (c) The mass of a family car 5 2 tonnes Exercises 4 Calculate the volume of a room of length 5m, width 10m and height 3m. 5 Using the information from page 1, calculate: (a) the volume of a human hair of length 20cm (b) the volume of the Earth. If nothing ever happened, would there be time? M01_IBPH_SB_HIGGLB_4426_U01.indd 2 29/6/10 11:48:25
15. 15. 3 Density By measuring the mass and volume of many different objects we find that if the objects are made of the same material, the ratio mass/volume is the same. This quantity is called the density. The unit of density is kgm23 . This is another derived unit. Examples include: Density of water 5 1.0 3 103 kgm23 Density of air 5 1.2kgm23 Density of gold 5 1.93 3 104 kgm23 Displacement So far all that we have modelled is the position of things and when events take place, but what if something moves from one place to another? To describe the movement of a body, we define the quantity displacement. This is the distance moved in a particular direction. The unit of displacement is the same as length: the metre. Example: Refering to the map in Figure 1.1: If you move from B to C, your displacement will be 5km north. If you move from A to B, your displacement will be 4km west. Summary of SI units The International System of units is the set of units that are internationally agreed to be used in science. It is still OK to use other systems in everyday life (miles, pounds, Fahrenheit) but in science we must always use SI. There are seven fundamental quantities. Base quantity Name Symbol length metre m mass kilogram kg time second s electric current ampere A thermodynamic temperature kelvin K amount of substance mole mol luminous intensity candela cd 6 Calculate the mass of air in a room of length 5m, width 10m and height 3m. 7 Calculate the mass of a gold bar of length 30cm, width 15cm and height 10cm. 8 Calculate the average density of the Earth. Exercises Figure 1.1 Displacements on a map. N B C 5km A M01_IBPH_SB_HIGGLB_4426_U01.indd 3 29/6/10 11:48:25
16. 16. 4 Physics and physical measurement1 All other SI units are derived units; these are based on the fundamental units and will be introduced and defined where relevant. So far we have come across just two. Derived quantity Symbol Base units volume m3 m 3 m 3 m density kgm23 kg ___________ m 3 m 3 m Uncertainty and error in measurement The SI system of units is defined so that we all use the same sized units when building our models of the physical world. However, before we can understand the relationship between different quantities, we must measure how big they are. To make measurements we use a variety of instruments. To measure length, we can use a ruler and to measure time, a clock. If our findings are to be trusted, then our measurements must be accurate, and the accuracy of our measurement depends on the instrument used and how we use it. Consider the following examples. Measuring length using a ruler Example 1 A good straight ruler marked in mm is used to measure the length of a rectangular piece of paper as in Figure 1.2. The ruler measures to within 0.5mm (we call this the uncertainty in the measurement) so the length incm is quoted to 2dp. This measurement is precise and accurate. Measurement1.2 Assessment statements 1.2.6 Describe and give examples of random and systematic errors. 1.2.7 Distinguish between precision and accuracy. 1.2.8 Explain how the effects of random errors may be reduced. 1.2.9 Calculate quantities and results of calculations to the appropriate number of significant figures. Estimating uncertainty When using a scale such as a ruler the uncertainty in the reading is 1 _2 of the smallest division. In this case the smallest division is 1mm so the uncertainty is 0.5mm. Figure 1.2 Length 5 6.40 60.05cm. 0 cm 1 2 3 4 5 6 7 Even this huge device at CERN has uncertainties. M01_IBPH_SB_HIGGLB_4426_U01.indd 4 29/6/10 11:48:27
17. 17. 5 Example 2 Figure 1.3 shows how a ruler with a broken end is used to measure the length of the same piece of paper. When using the ruler, you fail to notice the end is broken and think that the 0.5cm mark is the zero mark. This measurement is precise since the uncertainty is small but is not accurate since the value 6.90cm is wrong. Example 3 A cheap ruler marked only in 1 _2cm is used to measure the length of the paper as in Figure 1.4. These measurements are not precise but accurate, since you would get the same value every time. Example 4 In Figure 1.5, a good ruler is used to measure the maximum height of a bouncing ball. Even though the ruler is good it is very difficult to measure the height of the bouncing ball. Even though you can use the scale to within 0.5mm, the results are not precise (may be about 60.2cm). However, if you do enough runs of the same experiment, your final answer could be accurate. Errors in measurement There are two types of measurement error random and systematic. Random error If you measure a quantity many times and get lots of slightly different readings then this called a random error. For example, when measuring the bounce of a ball it is very difficult to get the same value every time even if the ball is doing the same thing. Systematic error This is when there is something wrong with the measuring device or method. Using a ruler with a broken end can lead to a zero error as in Example 2 above. Even with no random error in the results, youd still get the wrong answer. Reducing errors To reduce random errors you can repeat your measurements. If the uncertainty is truly random, they will lie either side of the true reading and the mean of these values will be close to the actual value. To reduce a systematic error you need to find If you measure the same thing many times and get the same value, then the measurement is precise. If the measured value is close to the expected, then the measurement is accurate. If a football player hit the post 10 times in a row when trying to score a goal, you could say the shots are precise but not accurate. Figure 1.5 Height 5 5.0 6 0.2cm. Figure 1.3 Length 5 6.90 6 0.05cm. Figure 1.4 Length 5 6.5 6 0.25cm. It is not possible to measure anything exactly. This is not because our instruments are not exact enough but because the quantities themselves do not exist as exact quantities. 1 2 3 4 5 6 7 0 cm 1 2 3 4 5 6 7 cm 1234567 M01_IBPH_SB_HIGGLB_4426_U01.indd 5 29/6/10 11:48:27
18. 18. 6 Physics and physical measurement1 out what is causing it and correct your measurements accordingly. A systematic error is not easy to spot by looking at the measurements, but is sometimes apparent when you look at the graph of your results or the final calculated value. Measurement in practice Approximately one quarter of this course will be taken up with practical work, where you will be measuring quantities and doing calculations. There are certain accepted ways of handling data and the uncertainties they contain. In the following section we will go through the way to make tables, draw graphs and handle uncertainties. If you follow this method you will gain good marks in the internally assessed part of the course. To illustrate the method we will consider a simple experiment. You are presented with 6 metal cubes and asked to find the density of the metal. The apparatus supplied is a good ruler and an electronic balance. The ruler has millimetre divisions and the balance measures down to 0.1g. Density is defined as mass/volume, so to find density we must measure mass and volume. As the samples are cubes we only need to measure one side. We can then find the volume by cubing this value. Repeating measurements Each cube has a different side length and mass. To make sure each mass is connected to the correct side length we put our data into a table; it is also simpler if we use a spreadsheet to perform calculations. The cubes are not perfectly uniform so the side length depends upon which length we choose. To reduce the uncertainty in this measurement we measure the cube four times and find the average side length. However, if we measure the mass of the cube we get exactly the same measurement each time and there is therefore no point in repeating this measurement. Mass/g 60.1g Length 1/cm 60.05cm Length 2/cm 60.05cm Length 3/cm 60.05cm Length 4/cm 60.05cm 124.1 2.40 2.30 2.50 2.40 235.2 3.00 3.10 2.90 3.00 344.0 3.40 3.30 3.40 3.50 463.2 3.70 3.80 3.60 3.70 571.2 4.00 4.10 3.90 4.00 660.0 4.20 4.30 4.10 4.20 Note: The uncertainty in each length measurement is 0.05cm. However the actual uncertainty is greater as the spread of values demonstrates. Examiners hint: Its OK to use nonSI units such as grams when collecting data. However, your final result (density) should be in SI units. Examiners hint: The number of decimal places in the data should be consistent with the uncertainty. It would be wrong to write 2.4000cm since the uncertainty is 6 0.05cm Table 1.1 Collecting data1.3 Assessment statements 1.2.10 State uncertainties as absolute, fractional and percentage uncertainties. 1.2.11 Determine the uncertainties in results. M01_IBPH_SB_HIGGLB_4426_U01.indd 6 29/6/10 11:48:27
19. 19. 7 This is calculated using the formula ((max length)3 2 (min length)3 )/2 This is found by calculating (max length 2 (min length)/2 Processing data In this example we are first going to find the average side length and then the volume. If we look at the data we can see that the spread of data is more than the uncertainty in the ruler scale so we will also calculate the uncertainty in these values. When we know that, we can find out the uncertainty in the volume. This is found by calculating the maximum and minimum volumes using the maximum and minimum values for length. Even in this simple experiment there will be a lot of calculations, so it is advisable to use a spreadsheet (such as Excel) for doing this. Mass/g 60.1g Mean length/cm Uncertainty in length/6cm Volume/cm3 Uncertainty in volume/6cm3 124.1 2.4 0.1 14 2 235.2 3.0 0.1 27 3 344.0 3.4 0.1 39 3 463.2 3.7 0.1 51 4 571.2 4.0 0.1 64 5 660.0 4.2 0.1 74 5 You could simply calculate the density of the metal for each cube and find the average but there are many advantages to using a graphical method. In this example, we know that the mass and volume are related by the equation m 5 V where is the density. This means that mass is proportional to volume, so plotting a graph of mass (on the y-axis) against volume (on the x-axis) will give a straight line. The gradient of the line will be the density and the y-intercept will be zero. Because there are uncertainties in the data, we dont know exactly where to plot the line; for this reason we plot error bars on each point as shown in Figure 1.6. This graph has been plotted with a computer program (e.g. Graphical Analysis by Vernier) that automatically plots the best fit line and the error bars. The general equation of a straight line is y 5 mx 1 c Where m is the gradient and c is the y-intercept. Any equation that has the same form will also be linear. Examiners hint: It is well worth learning how to use a spreadsheet programme for analysing data. Examiners hint: The number of decimal places in the data must not exceed the uncertainty and the uncertainty has been rounded off to 1significant figure. Examiners hint: As with spreadsheets, it is also worth practising with graph plotting software. Presenting processed data1.4 Assessment statements 1.2.12 Identify uncertainties as error bars in graphs. 1.2.13 State random uncertainty as an uncertainty range (6) and represent it graphically as an error bar. 1.2.14 Determine the uncertainties in the gradient and intercepts of a straight-line graph. Table 1.2 M01_IBPH_SB_HIGGLB_4426_U01.indd 7 29/6/10 11:48:28
20. 20. 8 Physics and physical measurement1 Uncertainties in gradients We can see from Figure 1.6 that the line shown is not the only straight line that can be drawn through the error bars; there are in fact a whole range of them. Using the steepest and least steep lines that we can draw will give us the uncertainty in the gradient. The computer program also enables us to do that, as shown in Figure 1.7. The maximum gradient is 10.1 and the minimum is 8.1 so the uncertainty is (10.1 2 8.1) __________ 2 5 1gcm23 So our final result is that the density of the metal in SI units is 9000 61000kgm23 . Note the number of significant figures is reduced so that it is consistent with the uncertainty. Figure 1.6 Graph of mass vs volume with error bars As we can see, the gradient or slope of the line is 8.999gcm23 ; this is the density of the metal. Figure 1.7 Graph of mass vs volume with steepest and least steep lines. 200 400 600 20 40 60 80 Volume/cm3 Mass/g Linear Fit for: Data SetMass m aV b a(Slope): 8.999 g/cm3 b (Y-intercept): 0.00g Correlation: 0.9998 RMSE: 4.991 To find out more about graph plotting visit www.heinemann. co.uk/hotlinks, enter the express code 4426P and click on Weblink1.1. 200 400 600 20 40 60 80 Volume/cm3 Mass/g Linear Fit for: Data SetMass m aV b a(Slope): 8.999 g/cm3 b (Y-intercept): 0.00 g Correlation: 0.9998 RMSE: 4.991 Manual Fit for: Data SetMass m aV b a(Slope): 8.099 g/cm3 b (Y-intercept): 23.16 g Manual Fit for: Data SetMass m aV b a(Slope): 10.10 g/cm3 b (Y-intercept): 35.34 g M01_IBPH_SB_HIGGLB_4426_U01.indd 8 29/6/10 11:48:28
21. 21. 9 If we look up the density of metals we find that the density of copper is 8920kgm23 . This is only 80kgm23 less than our value. We can therefore conclude that within the uncertainties of our experiment the cubes could be made of copper. Percentage uncertainties In the example above, the uncertainties were expressed as 61000kgm23 . This is called an absolute uncertainty. Uncertainties can also be expressed as a simple percentage. In this case the percentage uncertainty would be (1000____ 9000 )3 100 5 11% When you multiply values, you can find the uncertainty of the result by adding the percentage uncertainties. However when dealing with tables of data, it is simpler to use the method described previously. So if the uncertainty in length is 4%, the uncertainty in volume is 3 3 4% 5 12%. Other information from graphs A graph is not only a way to find a value, it also gives us information about the validity of the data. Here are some examples. An outlier If you have made a mistake, it will show on the graph as an outlier. This is a point that does not fit the line, like the one in Figure 1.8. Examiners hint: Even though you use absolute uncertainties in all your practical work you might be asked how to manipulate percentage uncertainties in the exam. 9 To measure the volume of an object, two lengths l1 and l2 are measured. l1 5 10.25 6 0.05cm l2 5 15.45 6 0.05cm Calculate: (a) the % uncertainty in l1 (b) the % uncertainty in l2 (c) the area of the object (d) the % uncertainty in the area. Exercise 200 400 600 20 40 60 80 Volume/cm3 Mass/g Linear Fit for: Data SetMass Figure 1.8 Graph with an outlier. M01_IBPH_SB_HIGGLB_4426_U01.indd 9 29/6/10 11:48:28
22. 22. 10 Physics and physical measurement1 A non-zero intercept If the intercept is supposed to be zero but isnt, this could be due to a systematic error. For example, if all the mass measurements had been 10g too big, we would get an intercept of 10g as in Figure 1.9. A non-linear trend Relationships in physics are not always linear. This is shown by the graph in Figure1.10, where the straight line does not fit, but there is a clear relationship. So far we have dealt with six different quantities: Length Time Mass Volume Density Displacement All of these quantities have a size, but displacement also has a direction. Quantities that have size and direction are vectors and those with only size are scalars; all quantities are either vectors or scalars. It will be apparent why it is important to make this distinction when we add displacements together. Vectors and scalars1.5 Assessment statements 1.3.1 Distinguish between vector and scalar quantities, and give examples of each. 1.3.2 Determine the sum or difference of two vectors by a graphical method. 1.3.3 Resolve vectors into perpendicular components along chosen axes. 0 100 10 Figure 1.10 Graph with a non-linear trend the line does not pass through the error bars. Figure 1.9 A non-zero intercept. 0 10 20 30 40 50 3 5 7 Length/m Mass/kg Scalar A quantity with magnitude only. Vector A quantity with magnitude and direction. Figure 1.11 Displacements shown on a map. N B C 5km A M01_IBPH_SB_HIGGLB_4426_U01.indd 10 29/6/10 11:48:29
23. 23. 11 Example Consider two displacements one after another as shown in Figure 1.11. Starting from A walk 4km west to B, then 5km north to C. The total displacement from the start is not 5 1 4 but can be found by drawing a line from A to C. We will find that there are many other vector quantities that can be added in the same way. Addition of vectors Vectors can be represented by drawing arrows. The length of the arrow is proportional to the magnitude of the quantity and the direction of the arrow is the direction of the quantity. To add vectors the arrows are simply arranged so that the point of one touches the tail of the other. The resultant vector is found by drawing a line joining the free tail to the free point. Example Figure 1.11 is a map illustrating the different displacements. We can represent the displacements by the vectors in Figure 1.12. Calculating the resultant: If the two vectors are at right angles to each other then the resultant will be the hypotenuse of a right-angled triangle. This means that we can use simple trigonometry to relate the different sides. You will find cos, sin and tan buttons on your calculator. These are used to calculate unknown sides of right-angled triangles. Sin 5 Opposite __________ Hypotenuse Cos 5 Adjacent __________ Hypotenuse Tan 5 Opposite ________ Adjacent Figure 1.12 Vector addition. Figure 1.13 Examiners hint: Not all vectors add up to give right-angled triangles but as these are the easy ones to solve we will consider only these in this course. Resultant 4km 5km Adjacent Hypotenuse Opposite M01_IBPH_SB_HIGGLB_4426_U01.indd 11 29/6/10 11:48:30
24. 24. 12 Physics and physical measurement1 Vector symbols To show that a quantity is a vector we can write it in a special way. In textbooks this is often in bold (A) but when you write you can put an arrow on the top. In physics texts the vector notation is often left out. This is because if we know that the symbol represents a displacement, then we know it is a vector and dont need the vector notation to remind us. Exercise 10 Use your calculator to find x in the following triangles. (a) (b) (c) (d) x 55 3cm 50 x 4cm 30 6cm x 20 x 3cm Pythagoras The most useful mathematical relationship for finding the resultant of two perpendicular vectors is Pythagoras theorem. Hypotenuse2 5 adjacent2 1 opposite2 Using trigonometry to solve vector problems Once the vectors have been arranged point to tail it is a simple matter of applying the trigonometrical relationships to the triangles that you get. Exercise 11 Use Pythagorastheorem to find the hypotenuse in the following examples. (a) (b) (c) (d) 3cm 4cm 4cm 4cm 6cm 2cm 2cm 3cm Exercises Draw the vectors and solve the following problems using Pythagorastheorem. 12 A boat travels 4km west followed by 8km north. What is the resultant displacement? 13 A plane flies 100km north then changes course to fly 50km east. What is the resultant displacement? M01_IBPH_SB_HIGGLB_4426_U01.indd 12 29/6/10 11:48:31
25. 25. 13 Vectors in one dimension In this course we will often consider the simplest examples where the motion is restricted to one dimension, for example a train travelling along a straight track. In examples like this there are only two possible directions forwards and backwards. To distinguish between the two directions,we give them different signs (forward 1 and backwards 2). Adding vectors is now simply a matter of adding the magnitudes, with no need for complicated triangles. Worked example If a train moves 100m forwards along a straight track then 50m back, what is its final displacement? Solution Figure 1.15 shows the vector diagram. The resultant is clearly 50m forwards. Subtracting vectors Now we know that a negative vector is simply the opposite direction to a positive vector, we can subtract vector B from vector A by changing the direction of vector B and adding it to A. A 2 B 5 A 1 (2B) Taking components of a vector Consider someone walking up the hill in Figure 1.17. They walk 5km up the slope but want to know how high they have climbed rather than how far they have walked. To calculate this they can use trigonometry. Height 5 5 3 sin 30 The height is called the vertical component of the displacement. The horizontal displacement can also be calculated. Horizontal displacement 5 5 3 cos 30 This process is called taking components of a vector and is often used in solving physics problems. Examiners hint: There isnt really any need to draw vector diagrams when doing one-dimensional problems. However, you must never forget that the sign gives the direction. Which direction is 1? You can decide for yourself which direction you want to be positive but generally we follow the convention: Right 1 Left 2 North 1 South 2 Up 1 Down 2 veve Figure 1.14 The train can only move forwards or backwards. Figure 1.15 Adding vectors in one dimension. 100m 50m A B A B A B Figure 1.17 5km up the hill but how high? 5km 30 A sin A A cos not next to the angle next to the angle Figure 1.18 An easy way to remember which is cos is to say that itsbecos its next to the angle. Figure 1.16 Subtracting vectors. M01_IBPH_SB_HIGGLB_4426_U01.indd 13 29/6/10 11:48:33
26. 26. 14 Physics and physical measurement1 Exercises 14 If a boat travels 10km in a direction 30 to the east of north, how far north has it travelled? 15 On his way to the South Pole, Amundsen travelled 8km in a direction that was 20 west of south. What was his displacement south? 16 A mountaineer climbs 500m up a slope that is inclined at an angle of 60 to the horizontal. How high has he climbed? 1 This question is about measuring the permittivity of free space 0. The diagram below shows two parallel conducting plates connected to a variable voltage supply.The plates are of equal areas and are a distance d apart. The charge Q on one of the plates is measured for different values of the potential difference V applied between the plates.The values obtained are shown in the table below.The uncertainty in the value of V is not significant but the uncertainty in Q is 610%. V / V Q / nC 6 10% 10.0 30 20.0 80 30.0 100 40.0 160 50.0 180 (a) Plot the data points opposite on a graph of V (x-axis) against Q (y-axis). (4) (b) By calculating the relevant uncertainty in Q, add error bars to the data points (10.0, 30) and (50.0, 180). (3) (c) On your graph, draw the line that best fits the data points and has the maximum permissible gradient. Determine the gradient of the line that you have drawn. (3) (d) The gradient of the graph is a property of the two plates and is known as capacitance. Deduce the units of capacitance. (1) Practice questions variable voltage supply d V Examiners hint: Question 1 is about analysing data. It is a typical Paper2 question. You dont have to know anything about permittivity to answer it. M01_IBPH_SB_HIGGLB_4426_U01.indd 14 29/6/10 11:48:33
27. 27. 15 The relationship between Q and V for this arrangement is given by the expression Q 5 0A ___ d V where A is the area of one of the plates. In this particular experiment A 5 0.20 6 0.05m2 and d 5 0.50 6 0.01mm. (e) Use your answer to (c) to determine the maximum value of 0 that this experiment yields. (4) (Total 15 marks) International Baccalaureate Organisation 2 A student measures a distance several times.The readings lie between 49.8cm and 50.2cm.This measurement is best recorded as A 49.8 6 0.2cm. B 49.8 6 0.4cm. C 50.0 6 0.2cm. D 50.0 6 0.4cm. (1) International Baccalaureate Organisation 3 The time period T of oscillation of a mass m suspended from a vertical spring is given by the expression T 5 2 __ m__ k where k is a constant. Which one of the following plots will give rise to a straight-line graph? A T2 against m B _ Tagainst __ m C T against m D _ Tagainst m (1) International Baccalaureate Organisation 4 The power dissipated in a resistor of resistance R carrying a current I is equal to I2 R.The value of I has an uncertainty of 62% and the value of R has an uncertainty of 610%. The value of the uncertainty in the calculated power dissipation is A 68%. B 612%. C 614%. D 620%. (1) International Baccalaureate Organisation 5 An ammeter has a zero offset error.This fault will affect A neither the precision nor the accuracy of the readings. B only the precision of the readings. C only the accuracy of the readings. D both the precision and the accuracy of the readings. (1) International Baccalaureate Organisation M01_IBPH_SB_HIGGLB_4426_U01.indd 15 29/6/10 11:48:33
28. 28. 16 Physics and physical measurement1 6 When a force F of (10.0 6 0.2)N is applied to a mass m of (2.0 6 0.1)kg, the percentage uncertainty attached to the value of the calculated acceleration F__ mis A 2%. B 5%. C 7%. D 10%. (1) International Baccalaureate Organisation 7 Which of the following is the best estimate, to one significant digit, of the quantity shown below? 3 8.1_______ _____ (15.9) A 1.5 B 2.0 C 5.8 D 6.0 (1) International Baccalaureate Organisation 8 Two objects X and Y are moving away from the point P.The diagram below shows the velocity vectors of the two objects. Which of the following velocity vectors best represents the velocity of object X relative to object Y? (1) International Baccalaureate Organisation 9 The order of magnitude of the weight of an apple is A 1024 N. B 1022 N. C 1N. D 102 N. (1) International Baccalaureate Organisation P Velocity vector for object Y Velocity vector for object X A B C D M01_IBPH_SB_HIGGLB_4426_U01.indd 16 29/6/10 11:48:33
29. 29. 17 2 Mechanics In Chapter 1, we observed that things move and now we are going to mathematically model that movement. Before we do that, we must define some quantities that we are going to use. Displacement and distance It is important to understand the difference between distance travelled and displacement. To explain this, consider the route marked out on the map shown in Figure 2.1 Displacement is the distance moved in a particular direction. The unit of displacement is the metre (m). Displacement is a vector quantity. On the map, the displacement is the length of the straight line from A to B, a distance of 5km west. (Note: since displacement is a vector you should always say what the direction is.) Distance is how far you have travelled from A to B. The unit of distance is also the metre. Distance is a scalar quantity. In this example, the distance travelled is the length of the path taken, which is about 10km. Sometimes this difference leads to a surprising result. For example, if you run all the way round a running track you will have travelled a distance of 400m but your displacement will be 0m. In everyday life, it is often more important to know the distance travelled. For example, if you are going to travel from Paris to Lyon by road you will want to know that the distance by road is 450km, not that your final displacement will be Kinematics2.1 Assessment statements 2.1.1 Define displacement, velocity, speed and acceleration. 2.1.2 Explain the difference between instantaneous and average values of speed, velocity and acceleration. 2.1.3 Outline the conditions under which the equations for uniformly accelerated motion may be applied. 2.1.9 Determine relative velocity in one and in two dimensions. N B A 5km Figure 2.1 M02_IBPH_SB_HIGGLB_4426_U02.indd 17 29/6/10 11:50:22
30. 30. 18 Mechanics2 336km SE. However, in physics, we break everything down into its simplest units, so we start by considering motion in a straight line only. In this case it is more useful to know the displacement, since that also has information about which direction you have moved. Velocity and speed Both speed and velocity are a measure of how fast a body is moving, but velocity is a vector quantity and speed is a scalar. Velocity is defined as the displacement per unit time. velocity5 displacement ___________ time The unit of velocity is ms21 . Velocity is a vector quantity. Speed is defined as the distance travelled per unit time. speed5 distance_______ time The unit of speed is also ms21 . Speed is a scalar quantity. Average velocity and instantaneous velocity Consider travelling by car from the north of Bangkok to the south a distance of about 16km. If the journey takes 4 hours, you can calculate your velocity to be 16 __45 4kmh21 in a southwards direction. This doesnt tell you anything about the journey, just the difference between the beginning and the end (unless you managed to travel at a constant speed in a straight line). The value calculated is the average velocity and in this example it is quite useless. If we broke the trip down into lots of small pieces, each lasting only one second, then for each second the car could be considered to be travelling in a straight line at a constant speed. For these short stages we could quote the cars instantaneous velocity thats how fast its going at that moment in time and in which direction. Figure 2.2 Its not possible to take this route across Bangkok with a constant velocity. The bus in the photo has a constant velocity for a very short time. Exercise 1 Convert the following speeds into ms21 . (a) A car travelling at 100kmh21 (b) A runner running at 20kmh21 M02_IBPH_SB_HIGGLB_4426_U02.indd 18 29/6/10 11:50:23
31. 31. 19 Measuring velocity You can measure velocity with a photogate connected to a timer or computer. When a card passes through the gate it is sensed by the timer, switching it on or off. average velocity5 distance________________________________ time taken to travel between photogates instantaneous velocity5 length of card____________________________ time for card to pass through gate Velocity is relative When quoting the velocity of a body, it is important to say what the velocity is measured relative to. Consider the people in Figure 2.4 C measures the velocity of A to be 1ms21 but to B (moving on the truck towards C) the velocity of A is 29ms21 (B will see A moving away in a negative direction). You might think that A cant have two velocities, but he can velocity is relative. In this example there are two observers, B and C. Each observer has a different frame of reference. To convert a velocity, to Bs frame of reference, we must subtract the velocity of B relative to C; this is 10ms21 . So the velocity of A relative to B 5 1 2 10 5 29ms21 We can try the same with D who has a velocity of 21ms21 measured by C and 21 2 10 5 211ms21 measured by B. This also works in two dimensions as follows: A now walks across the road as illustrated by the aerial view in Figure 2.5. The velocity of A relative to C is 1ms21 north. The velocity of A relative to B can now be found by subtracting the vectors as shown in Figure 2.6. Photogate 1 Distance Photogate 2 Card Figure 2.3 Experimental set up for measuring velocity. Figure 2.4 Two observers measuring the same velocity. Figure 2.6 Subtracting vectors gives the relative velocity. Figure 2.5 A now walks across the road.. 1ms1 1ms110ms1 A DB C 10ms1 1ms1 B A C Examiners hint: Velocity is a vector. If the motion is in one dimension, the direction of velocity is given by its sign. Generally, right is positive and left negative. 2 An observer standing on a road watches a bird flying east at a velocity of 10ms21 . A second observer, driving a car along the road northwards at 20ms21 sees the bird. What is the velocity of the bird relative to the driver? Exercise M02_IBPH_SB_HIGGLB_4426_U02.indd 19 29/6/10 11:50:24
32. 32. 20 Mechanics2 Acceleration In everyday usage, the word accelerate means to go faster. However in physics: acceleration is defined as the rate of change of velocity. acceleration 5 change of velocity _______________ time The unit of acceleration is ms22 . Acceleration is a vector quantity. This means that whenever a body changes its velocity it accelerates. This could be because it is getting faster, slower or just changing direction. In the example of the journey across Bangkok, the car would have been slowing down, speeding up and going round corners almost the whole time, so it would have had many different accelerations. However, this example is far too complicated for us to consider in this course (and probably any physics course). For most of this chapter we will only consider the simplest example of accelerated motion, constant acceleration. Constant acceleration in one dimension In one-dimensional motion, the acceleration, velocity and displacement are all in the same direction. This means they can simply be added without having to draw triangles. Figure 2.7 shows a body that is starting from an initial velocity u and accelerating at a constant rate to velocity v in t seconds. The distance travelled in this time is s. Since the motion is in a straight line, this is also the displacement. Using the definitions already stated, we can write equations related to this example. Average velocity From the definition, the average velocity 5 displacement ___________ time So average velocity 5 s_ t (1) Since the velocity changes at a constant rate from the beginning to the end, we can also calculate the average velocity by adding the velocities and dividing by two. Average velocity 5 (u 1 v) _______ 2 (2) Acceleration Acceleration is defined as the rate of change of velocity. So a 5 (v 2 u) _______ t (3) We can use these equations to solve any problem involving constant acceleration. However, to make problem solving easier, we can derive two more equations by substituting from one into the other. Bodies When we refer to a body in physics we generally mean a ball not a human body. u time 0 time t va s Figure 2.7 A red ball is accelerated at a constant rate. M02_IBPH_SB_HIGGLB_4426_U02.indd 20 29/6/10 11:50:24
33. 33. 21 Equating equations (1) and(2) s_ t5 (u 1 v) _______ 2 so s5 (u 1 v) t ________ 2 (4) Rearranging (3) gives v 5 u 1 at If we substitute for v in equation (4) we get s 5 ut 1 1 _2at2 (5) Rearranging (3) again gives t 5 (v 2 u) _______a If t is now substituted in equation (4) we get v2 5 u2 1 2as (6) These equations are sometimes known as the suvat equations. If you know any 3 of suva and t you can find either of the other two in one step. Worked example 1 A car travelling at 10ms21 accelerates at 2ms22 for 5s. What is its displacement? Solution The first thing to do is draw a simple diagram like Figure 2.8. This enables you to see what is happening at a glance rather than reading the text. The next stage is to make a list of suvat. s5 ? u5 10ms21 v5 ? a5 2ms22 t5 5s To find s you need an equation that contains suat. The only equation with all 4 of these quantities is s 5 ut 1 1 _2at2 Using this equation gives: s5 10 3 5 1 1 _23 2 3 52 s5 75m The sign of displacement, velocity and acceleration We must not forget that displacement, velocity and acceleration are vectors. This means that they have direction. However, since this is a one-dimensional example, there are only two possible directions, forward and backward. We know which direction the quantity is in from the sign. A positive displacement means that the body has moved right. A positive velocity means the body is moving to the right. u 10ms1 time 0 time 5s a 2ms2 Figure 2.8 A simple diagram is always the best start. Examiners hint: You dont need to include units in all stages of a calculation, just the answer. suvat equations a 5 (v 2 u) ______ t s 5 (u 1 v) t _______ 2 s 5 ut 1 1 _2at2 v2 5 u2 1 2as M02_IBPH_SB_HIGGLB_4426_U02.indd 21 29/6/10 11:50:24
34. 34. 22 Mechanics2 A positive acceleration means that the body is either moving to the right and getting faster or moving to the left and getting slower. This can be confusing, so consider the following example. The car is travelling in a negative direction so the velocities are negative. u 5 210ms21 v 5 25ms21 t 5 5s The acceleration is therefore given by a 5 (v 2 u) _______ t5 25 2 210_________ 5 5 1ms22 The positive sign tells us that the acceleration is in a positive direction (right) even though the car is travelling in a negative direction (left). Example A body with a constant acceleration of 25ms22 is travelling to the right with a velocity of 20ms21 . What will its displacement be after 20s? s5 ? u5 20ms21 v5 ? a5 25ms22 t5 20s To calculate s we can use the equation s 5 ut 1 1 _2at2 s 5 20 3 20 1 1 _2(25) 3 202 5 400 2 1000 5 2600m This means that the final displacement of the body is to the left of the starting point. It has gone forward, stopped and then gone backwards. 5ms1 time 5s time 0 u 10ms1 Figure 2.9 Figure 2.10 The acceleration is negative so pointing to the left. 20ms1 5ms2 Exercises 3 Calculate the final velocity of a body that starts from rest and accelerates at 5ms22 for a distance of 100m. 4 A body starts with a velocity of 20ms21 and accelerates for 200m with an acceleration of 5ms22 . What is the final velocity of the body? 5 A body accelerates at 10ms22 reaching a final velocity of 20ms21 in 5s. What was the initial velocity of the body? g The acceleration due to gravity is not constant all over the Earth. 9.81ms22 is the average value. The acceleration also gets smaller the higher you go. However we ignore this change when conducting experiments in the lab since labs arent that high. To make the examples easier to follow, g 5 10ms22 is used throughout; you should only use this approximate value in exam questions if told to do so. Free fall motion2.2 Assessment statements 2.1.4 Identify the acceleration of a body falling in a vacuum near the Earths surface with the acceleration g of free fall. 2.1.5 Solve problems involving the equations of uniformly accelerated motion. 2.1.6 Describe the effects of air resistance on falling objects. M02_IBPH_SB_HIGGLB_4426_U02.indd 22 29/6/10 11:50:25
35. 35. 23 Although a car was used in one of the previous illustrations, the acceleration of a car is not usually constant, so we shouldnt use the suvat equations. The only example of constant acceleration that we see in everyday life is when a body is dropped. Even then the acceleration is only constant for a short distance. Acceleration of free fall When a body is allowed to fall freely we say it is in free fall. Bodies falling freely on the Earth fall with an acceleration of about 9.81ms22 . (It depends where you are.) The body falls because of gravity. For that reason we use the letter g to denote this acceleration. Since the acceleration is constant, we can use the suvat equations to solve problems. Measuring g Measuring g by timing a ball falling from different heights is a common physics experiment that you could well perform in the practical programme of the IB course. There are various different ways of doing this but a common method is to use a timer that starts when the ball is released and stops when it hits a platform. An example of this apparatus is shown in the photo. The distance travelled by the ball and the time taken are related by the suvat equation s 5 ut 1 1 _2at2 . This simplifies to s 5 1 _2at2 since the initial velocity is zero. This means that s is proportional to t2 so if you plot a graph of s against t2 you will get a straight line whose gradient is 1 _2g. In these calculations use g 5 10ms22 . 6 A ball is thrown upwards with a velocity of 30ms21 . What is the displacement of the ball after 2s? 7 A ball is dropped. What will its velocity be after falling 65cm? 8 A ball is thrown upwards with a velocity of 20ms21 . After how many seconds will the ball return to its starting point? Exercises The effect of air resistance If you jump out of a plane (with a parachute on) you will feel the push of the air as it rushes past you. As you fall faster and faster, the air will push upwards more and more until you cant go any faster. At this point you have reached terminal velocity. We will come back to this example after introducing forces. Free fall apparatus. Graphical representation of motion2.3 Assessment statements 2.1.7 Draw and analyse distancetime graphs, displacementtime graphs, velocitytime graphs and accelerationtime graphs. 2.1.8 Calculate and interpret the gradients of displacementtime graphs and velocitytime graphs, and the areas under velocitytime graphs and accelerationtime graphs. M02_IBPH_SB_HIGGLB_4426_U02.indd 23 29/6/10 11:50:26
36. 36. 24 Mechanics2 Graphs are used in physics to give a visual representation of relationships. In kinematics they can be used to show how displacement, velocity and acceleration change with time. Figure 2.11 shows the graphs for four different examples of motion. They are placed vertically since they all have the same time axis. Line A A body that is not moving. Displacement is always the same. Velocity is zero. Acceleration is zero. Line B A body that is travelling with a constant positive velocity. Displacement increases linearly with time. Velocity is a constant positive value. Acceleration is zero. Line C A body that has a constant negative velocity. Displacement is decreasing linearly with time. Velocity is a constant negative value. Acceleration is zero. Line D A body that is accelerating with constant acceleration. Displacement is increasing at a non-linear rate. The shape of this line is a parabola since displacement is proportional to t2 (s 5 ut 1 1 _2at2 ). Velocity is increasing linearly with time. Acceleration is a constant positive value. The best way to go about sketching graphs is to split the motion into sections then plot where the body is at different times; joining these points will give the displacementtime graph. Once you have done that you can work out the vt and at graphs by looking at the st graph rather than the motion. Gradient of displacementtime The gradient of a graph is change in y __________ change in x 5y___ x In the case of the displacementtime graph this will give gradient 5s___ t This is the same as velocity. So the gradient of the displacementtime graph equals the velocity. Using this information, we can see that line A in Figure 2.12 represents a body with greater velocity than line B and that since the gradient of line C is increasing, this must be the graph for an accelerating body. D B A C time displacement D B C time A velocity D time A B C acceleration Figure 2.11 Graphical representation of motion. C B A time displacement Figure 2.12 Examiners hint: You need to be able to figure out what kind of motion a body has by looking at the graphs sketch graphs for a given motion. M02_IBPH_SB_HIGGLB_4426_U02.indd 24 29/6/10 11:50:26
37. 37. 25 Instantaneous velocity When a body accelerates its velocity is constantly changing. The displacement time graph for this motion is therefore a curve. To find the instantaneous velocity from the graph we can draw a tangent to the curve and find the gradient of the tangent as shown in Figure 2.13. Area under velocitytime graph The area under the velocitytime graph for the body travelling at constant velocity v shown in Figure 2.14 is given by area 5 vt But we know from the definition of velocity that v 5 s___ t Rearranging gives s 5 vt so the area under a velocitytime graph gives the displacement. This is true not only for simple cases such as this but for all examples. Gradient of velocitytime graph The gradient of the velocitytime graph is given by v___ t . This is the same as acceleration. Area under accelerationtime graph The area under an accelerationtime graph in Figure 2.15 is given by at. But we know from the definition of acceleration that a 5 (v 2 u) _______ t Rearranging this gives v 2 u 5 at so the area under the graph gives the change in velocity. If you have covered calculus in your maths course you may recognise these equations: v 5 ds__ dt a 5 dv__ dt 5 d2 s___ dt2and s 5 vdt, v 5 adt s t time displacement Figure 2.13 Figure 2.15 Figure 2.14 t time velocity v t time acceleration a 9 Sketch a velocitytime graph for a body starting from rest and accelerating at a constant rate to a final velocity of 25ms21 in 10 seconds. Use the graph to find the distance travelled and the acceleration of the body. 10 Describe the motion of the body whose velocitytime graph is shown in Figure 2.16. What is the final displacement of the body? Exercises 10 10 time/s velocity/ms1 3 6 9 Figure 2.16 M02_IBPH_SB_HIGGLB_4426_U02.indd 25 29/6/10 11:50:27
38. 38. 26 Mechanics2 Example 1: the suvat example As an example let us consider the motion we looked at when deriving the suvat equations. Displacementtime The body starts with velocity u and travels to the right with constant acceleration, a for a time t. If we take the starting point to be zero displacement, then the displacementtime graph starts from zero and rises to s in t seconds. We can therefore plot the two points shown in Figure 2.18. The body is accelerating so the line joining these points is a parabola. The whole parabola has been drawn to show what it would look like the reason it is offset is because the body is not starting from rest. The part of the curve to the left of the origin tells us what the particle was doing before we started the clock. Velocitytime Figure 2.19 is a straight line with a positive gradient showing that the acceleration is constant. The line doesnt start from the origin since the initial velocity is u. The gradient of this line is (v 2 u) _______ twhich we know from the suvat equations is acceleration. The area under the line makes the shape of a trapezium. The area of this trapezium is 1 _2(v 1 u)t. This is the suvat equation for s. Accelerationtime The acceleration is constant so the accelerationtime graph is simply a horizontal line as shown in Figure 2.20. The area under this line is a 3 t which we know from the suvat equations equals (v 2 u). Example 2: The bouncing ball Consider a rubber ball dropped from some position above the ground A onto a hard surface B. The ball bounces up and down several times. Figure 2.21 shows the u time 0 time t va s s t displacement time Figure 2.18 Figure 2.19 Figure 2.21 Figure 2.17 A body with constant acceleration. Negative time Negative time doesnt mean going back in time it means the time before you started the clock. v u time velocity t Figure 2.20 a time acceleration t A A B B C D time displacement M02_IBPH_SB_HIGGLB_4426_U02.indd 26 29/6/10 11:50:28
39. 39. 27 displacementtime graph for 4 bounces. From the graph we see that the ball starts above the ground then falls with increasing velocity (as deduced by the increasing negative gradient). When the ball bounces at B the velocity suddenly changes from negative to positive as the ball begins to travel back up. As the ball goes up, its velocity gets less until it stops at C and begins to fall again. Example 3: A ball falling with air resistance Figure 2.22 represents the motion of a ball that is dropped several hundred metres through the air. It starts from rest and accelerates for some time. As the ball accelerates, the air resistance gets bigger, which prevents the ball from getting any faster. At this point the ball continues with constant velocity. 11 By considering the gradient of the displacementtime graph in Figure 2.21 plot the velocity time graph for the motion of the bouncing ball. Exercise 12 By considering the gradient of the displacementtime graph plot the velocitytime graph for the motion of the falling ball. Exercise time displacement Figure 2.22 Projectile motion2.4 Assessment statements 9.1.1 State the independence of the vertical and the horizontal components of velocity for a projectile in a uniform field. 9.1.2 Describe and sketch the trajectory of projectile motion as parabolic in the absence of air resistance. 9.1.3 Describe qualitatively the effect of air resistance on the trajectory of a projectile. 9.1.4 Solve problems on projectile motion. To view a simulation that enables you to plot the graphs as you watch the motion, visit www.heinemann.co.uk/hotlinks, enter the express code 4426P and click on Weblink 2.1. M02_IBPH_SB_HIGGLB_4426_U02.indd 27 29/6/10 11:50:28
40. 40. 28 Mechanics2 We all know what happens when a ball is thrown; it follows a curved path like the one in the photo below. We can see from this photo that the path is parabolic, and later we will show why that is the case. Modelling projectile motion All examples of motion up to this point have been in one dimension but projectile motion is two-dimensional. However, if we take components of all the vectors vertically and horizontally, we can simplify this into two simultaneous one-dimensional problems. The important thing to realise is that the vertical and horizontal components are independent of each other; you can test this by dropping a stone off a cliff and throwing one forward at the same time, they both hit the bottom together. The downward motion is not altered by the fact that one is also moving forward. Consider a ball that is projected at an angle to the horizontal, as shown in Figure2.23. We can split the motion into three parts, beginning, middle and end, and analyse the vectors representing displacement, velocity and time at each stage. Note that since the path is symmetrical, the motion on the way down is the same as the way up. A stroboscopic photograph of a projected ball. A B C v R Range h Max height v g g Figure 2.23 A projectile launched at an angle . M02_IBPH_SB_HIGGLB_4426_U02.indd 28 29/6/10 11:50:29
41. 41. 29 Horizontal components At A (time 5 0) At B (time 5 t__ 2 ) At C (time 5 t) Displacement 5 zero Velocity 5 vcos Acceleration 5 0 Displacement 5 R__ 2 Velocity 5 vcos Acceleration 5 0 Displacement 5 R Velocity 5 vcos Acceleration 5 0 Vertical components At A At B At C Displacement 5 zero Velocity 5 vsin Acceleration 5 2g Displacement 5 h Velocity 5 zero Acceleration 5 2g Displacement 5 zero Velocity 5 2vsin Acceleration 5 2g We can see that the vertical motion is constant acceleration and the horizontal motion is constant velocity. We can therefore use the suvat equations. suvat for horizontal motion Since acceleration is zero there is only one equation needed to define the motion suvat A to C Velocity 5 v 5 s_ t R 5 vcos t suvat for vertical motion When considering the vertical motion it is worth splitting the motion into two parts. suvat At B At C s 5 1 _2 (u 1 v)t v2 5 u2 1 2as s 5 ut 1 1 _2 at2 a 5 v 2 u______ t h 5 1 _2 (vsin)t__ 2 0 5 v2 sin2 2 2gh h 5 vsin t 2 1 _2 g(t__ 2 )2 g 5 vsin 2 0_________ t__ 2 0 5 1 _2 (vsin 2 vsin)t (2vsin)2 5 (vsin)2 2 0 0 5 vsin t 2 1 _2 gt2 g 5 vsin 2 2vsin_______________ t Some of these equations are not very useful since they simply state that 0 5 0. However we do end up with three useful ones (highlighted) : R 5 vcos t (1) 0 5 v2 sin2 2 2gh or h 5 v2 sin2 _______ 2g (2) 0 5 vsin t 2 1 _2gt2 or t 5 2vsin______g (3) Solving problems In a typical problem you will be given the magnitude and direction of the initial velocity and asked to find either the maximum height or range. To calculate h you can use equation (2) but to calculate R you need to find the time of flight so must use (3) first (you could also substitute for t into equation (1) to give a fourth equation but maybe we have enough equations already). You do not have to remember a lot of equations to solve a projectile problem. If you understand how to apply the suvat equations to the two components of the projectile motion, you only have to remember the suvat equations (and they are in the databook). Parabolic path Since the horizontal displacement is proportional to t the path has the same shape as a graph of vertical displacement plotted against time. This is parabolic since the vertical displacement is proportional to t2 . Maximum range For a given value of v the maximum range is when vcos t is a maximum value. Now t 5 2vsin______g . If we substitute this for t we get R 5 2v2 cos sin___________g . This is a maximum when cos sin is maximum, which is when 5 45. M02_IBPH_SB_HIGGLB_4426_U02.indd 29 29/6/10 11:50:30
42. 42. 30 Mechanics2 Worked example 1 A ball is thrown at an angle of 30 to the horizontal at a speed of 20ms21 . Calculate its range and the maximum height reached. 2 A ball is thrown horizontally from a cliff top with a horizontal speed of 10ms21 . If the cliff is 20m high what is the range of the ball? Solution 1 First, as always, draw a diagram, including labels defining all the quantities known and unknown. Now we need to find the time of flight. If we apply s 5 ut 1 1 _2at2 to the whole flight we get t 5 2vsin______g5 (2 3 20 3 sin30) _______________ 10 5 2s We can now apply s 5 vt to the whole flight to find the range: R 5 vcost 5 20 3 cos30 3 2 5 34.6m Finally to find the height, we use s 5 ut 1 1 _2at2 to the vertical motion, but remember, this is only half the complete flight so the time is 1s. h 5 vsint21 _2gt2 5 20 3 sin30 3 1 21 _23 10 3 12 5 10 2 5 5 5m 2 This is an easy one since there arent any angles to deal with. The initial vertical component of the velocity is zero and the horizontal component is 10ms21 . To calculate the time of flight we apply s 5 ut 1 1 _2at2 to the vertical component. Knowing that the final displacement is 220m this gives 220m 5 0 2 1 _2gt2 so t 5 ________ (2 3 20) ________ 10 5 2s We can now use this value to find the range by applying the formula s 5 vt to the horizontal component: R 5 10 3 2 5 20m Exercises 13 Calculate the range of a projectile thrown at an angle of 60 to the horizontal with velocity 30ms21 . 14 You throw a ball at a speed of 20ms21 . (a) At what angle must you throw it so that it will just get over a wall that is 5m high? (b) How far away from the wall must you be standing? 15 A gun is aimed so that it points directly at the centre of a target 200m away. If the bullet travels at 200ms21 how far below the centre of the target will the bullet hit? 16 If you can throw a ball at 20ms21 what is the maximum distance you can throw it? If you have ever played golf you will know it is not true that the maximum range is achieved with an angle of 45, its actually much less. This is because the ball is held up by the air like an aeroplane is. In this photo Alan Shepard is playing golf on the moon. Here the maximum range will be at 45. 10ms1 20m R 20ms1 30 h R To view a simulation of projectile motion, visit www.heinemann.co.uk/hotlinks, enter the express code 4426P and click on Weblink 2.2. M02_IBPH_SB_HIGGLB_4426_U02.indd 30 29/6/10 11:50:30
43. 43. 31 Projectile motion with air resistance In all the examples above we have ignored the fact that the air will resist the motion of the ball. The actual path of a ball including air resistance is likely to be as shown in Figure 2.24. Notice both the height and range are less. It is also no longer a parabola the way down is steeper than the way up. Forces From experience, we know that things dont seem to move unless we push them, so movement is related to pushing. In this next section we will investigate this relationship. What is a force? A force is simply a push or a pull. The unit of force is the newton (N). Force is a vector quantity. You might believe that there are hundreds of different ways to push or pull an object but there are actually surprisingly few. 1. Tension If you attach a rope to a body and pull it, the rope is in tension. This is also the name of the force exerted on the body. Figure 2.24 Figure 2.25 The force experienced by the block is tension, T. Without air resistance With air resistance The size of one newton If you hold an object of mass 100g in your hand then you will be exerting an upward force of about one newton (1N). Forces and dynamics2.5 Assessment statements 2.2.1 Calculate the weight of a body using the expression W 5 mg. 2.2.2 Identify the forces acting on an object and draw free body diagrams representing the forces acting. 2.2.3 Determine the resultant force in different situations. T M02_IBPH_SB_HIGGLB_4426_U02.indd 31 29/6/10 11:50:31
44. 44. 32 Mechanics2 2. Normal force Whenever two surfaces are in contact, there will be a force between them (if not then they are not in contact). This force acts at right angles to the surface so is called the normal force. 3. Gravitational force We know that all objects experience a force that pulls them downwards; we call this force the weight. The direction of this force is always towards the centre of the Earth. The weight of a body is directly proportional to the mass of the body. W 5 mg where g 5 the acceleration of free fall. You will discover why this is the case later in the chapter. 4. Friction force Whenever two touching surfaces move, or attempt to move, relative to each other, there is a force that opposes the motion. This is called frictional force. The size of this force is dependent on the material of the surfaces and how much force is used to push them together. 5. Upthrust Upthrust is the name of the force experienced by a body immersed in a fluid (gas or liquid). This is the force that pushes up on a boat enabling it to float in water. The size of this force is equal to the weight of fluid displaced by the boat. Figure 2.26 The man pushes the block with his hands. The force is called the normal force, N. N Figure 2.27 This box is pulled downwards by gravity. We call this force the weight, W. Figure 2.28 This box sliding along the floor will slow down due to the friction, F, between it and the floor. Figure 2.29 Upthrust U depends on how much water is displaced. W U u Where to draw forces It is important that you draw the point of application of the forces in the correct place. Notice where the forces are applied in these diagrams. F v M02_IBPH_SB_HIGGLB_4426_U02.indd 32 29/6/10 11:50:32
45. 45. 33 6. Air resistance Air resistance is the force that opposes the motion of bodies through the air. This force is dependent on the speed, size and shape of the body. Free body diagrams Problems often involve more than one body and more than one force. To keep things simple we always draw each body separately and only the forces acting on that body, not the forces that body exerts on something else. This is called a free body diagram. A good example of this is a block resting on a ramp as shown in Figure 2.30. The block will also exert a force on the slope but this is not shown, since it is a free body diagram of the block not the ramp. Another common example that we will come across many times is a mass swinging on the end of a rope, as shown in Figure 2.31. Speed skiers wear special clothes and squat down like this to reduce air resistance. W F N Figure 2.30 Figure 2.31 W T Fixed end 17 Draw free body diagrams for (a) a box resting on the floor (b) the examples shown below i ii (c) a free fall parachutist falling through the air (d) a boat floating in water. Exercise M02_IBPH_SB_HIGGLB_4426_U02.indd 33 29/6/10 11:50:34
46. 46. 34 Mechanics2 Adding forces Force is a vector quantity, so if two forces act on the same body you must add them vectorially as with displacements and velocities. Examples 1 A body is pulled in two opposing directions by two ropes as shown in Figure2.32. The resultant force acting is the vector sum of the forces. The sum is found by arranging the vectors point to tail. This gives a resultant of 2N to the left. 2 If a body is pulled by two perpendicular ropes as in Figure 2.33, then the vector addition gives a triangle that can be solved by Pythagoras. Balanced forces If the resultant force on a body is zero, the forces are said to be balanced. For example, if we add together the vectors representing the forces on the box in Figure 2.34 then we can see that they add up to zero. The forces are therefore balanced. This can lead to some complicated triangles so it is easier to take components of the forces; if the components in any two perpendicular directions are balanced, then the forces are balanced. Figure 2.35 shows how this would be applied to the same example. To make things clear, the vectors have been drawn away from the box. Vertical components add up to zero. Fsin 1 Ncos 2 W 5 0 W 5 Fsin 1 Ncos Horizontal components add up to zero. 0 1 Fcos 2 Nsin 5 0 Fcos 5 Nsin So we can see that forces up 5 forces down forces left 5 forces right 4N 3N 5N 4N 3N Figure 2.33 6N 4N 6N 4N 2N Figure 2.32 Exercise 18 Find the resultant force in the following examples: (a) (b)10N 10N 10N 3N 5N Figure 2.34 N F W Figure 2.35 W F N M02_IBPH_SB_HIGGLB_4426_U02.indd 34 29/6/10 11:50:35
47. 47. 35 2.6 Newtons laws of motion We now have the quantities to enable us to model motion and we have observed that to make something start moving we have to exert a force but we havent connected the two. Newtons laws of motion connect motion with its cause. In this course there are certain fundamental concepts that everything else rests upon, Newtons laws of motion are among the most important of these. Newtons first law A body will remain at rest or moving with constant velocity unless acted upon by an unbalanced force. To put this the other way round, if the forces on a body are unbalanced, then it will not be at rest or moving with constant velocity. If the velocity is not constant then it is accelerating. 19 A ball of weight 10N is suspended on a string and pulled to one side by another horizontal string as shown in Figure 2.36. If the forces are balanced: (a) write an equation for the horizontal components of the forces acting on the ball (b) write an equation for the vertical components of the forces acting on the ball (c) use the second equation to calculate the tension in the upper string, T (d) use your answer to (c) plus the first equation to find the horizontal force F. 20 The condition for the forces to be balanced is that the sum of components of the forces in any two perpendicular components is zero. In thebox on a rampexample the vertical and horizontal components were taken. However, it is sometimes more convenient to consider components parallel and perpendicular to the ramp. Consider the situation in Figure 2.37. If the forces on this box are balanced: (a) write an equation for the components of the forces parallel to the ramp (b) write an equation for the forces perpendicular to the ramp (c) use your answers to find the friction (F) and normal force (N). 21 A rock climber is hanging from a rope attached to the cliff by two bolts as shown in Figure 2.38. If the forces are balanced (a) write an equation for the vertical component of the forces on the knot (b) write an equation for the horizontal forces exerted on the knot (c) calculate the tension T in the ropes joined to the bolts. The result of this calculation shows why ropes should not be connected in this way. Exercises F T 10N 30 Figure 2.36 Figure 2.37 N F 50N 30 Figure 2.38 600N knot 80 80 TT Assessment statements 2.2.4 State Newtons first law of motion. 2.2.5 Describe examples of Newtons first law. 2.2.6 State the condition for translational equilibrium. 2.2.7 Solve problems involving translational equilibrium. Using laws in physics A law in physics is a very useful tool. If applied properly, it enables us to make a very strong argument that what we say is true. If asked will a box move?you can say that you think it will and someone else could say it wont. You both have your opinions and you would then argue as to who is right. However, if you say that Newtons law says it will move, then you have a much stronger argument (assuming you have applied the law correctly). M02_IBPH_SB_HIGGLB_4426_U02.indd 35 29/6/10 11:50:35
48. 48. 36 Mechanics2 We know from experience that things dont start moving unless we push them, but its not obvious from observation that things will continue moving with constant velocity unless acted upon by an unbalanced force. Usually, if you give an object a push it moves for a bit and then stops. This is because of friction. It would be a very different world without friction, as everyone would be gliding around with constant velocity, only able to stop themselves when they grabbed hold of something. Friction not only stops things moving but enables them to get going. Ifyou stood in the middle of a friction-free room, you wouldnt be able to move. Itis the friction between your feet and the floor that pushes you forward when you try to move your feet backwards. Examples 1 Mass on a string If a mass is hanging at rest on the end of a string as in Figure 2.39 then Newtons first law says the forces must be balanced. This means the Force up 5 Force down. T 5 mg 2 Car travelling at constant velocity If the car in Figure 2.40 is travelling at constant velocity, then Newtons first law says the forces must be balanced. Force up 5 Force down N 5 mg (not drawn on diagram) Force left 5 Force right F 5 Fa 3 The parachutist If the free fall parachutist in Figure 2.41 descends at a constant velocity then Newtons first law says that the forces must be balanced. Force up 5 Force down Fa 5 mg Translational equilibrium If all the forces on a body are balanced, the body is said to be in translational equilibrium. The bodies in the previous three examples were all therefore in translational equilibrium. mg T Figure 2.39 Figure 2.40 Notice that the friction ac