Topic 3 Force and Energy

� INTRODUCTION

We have discussed motion in terms of velocity and acceleration. But why do objects move? What makes an object which is at rest move? What causes a car to accelerate or decelerate? Each case mentioned involves force. We experience force in most of the things that we do. Any push or pull on an object requires force, like pushing a trolley at the supermarket or pulling your luggage bag at the airport. Force is also involved when a motor lifts an elevator or a hammer hits a nail or even when the leaves of a tree move when the wind blows. An object falls to the ground due to a certain force called the force of gravity. Force, however, does not always give rise to motion. You may have experienced pushing a car very hard and it still does not move!

By the end of this topic, you should be able to:

1. Define force and its types;

2. Describe six effects of force;

3. State the relationship between force, mass and acceleration (F = ma);

4. Describe gravity;

5. Relate work to power;

6. Differentiate between potential energy, kinetic energy and energy conservation; and

7. Discuss simple and compound machines.

LEARNING OUTCOMES

TTooppiicc

33 � Force and

Energy

TOPIC 3 FORCE AND ENERGY �

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In order to understand force, let us look at two important elements associated with it � pushing and pulling.

Whenever there is iinteraction between two objects, there is a force upon each of the objects. When the interaction ceases, the two objects no longer experience the force. Forces only exist as a result of an interaction. Do you know that there are many forces acting on you all of the time? One example is the force of gravity. You might not be aware of this but this force exists with you all the time. This can be felt if you are falling � you feel and sense that something is pulling you down. On the other hand, if you are standing perfectly still on the floor, the floor is pushing up on you just as hard as gravity is pulling you down!

TYPES OF FORCES

Now, let us get to know how many types of forces there are. There are many types of forces around us. However, in this module, we will only discuss four �frictional, mmagnetic, ggravitational and eelectrostatic. Let us look at them one by one.

3.1.1 Frictional Force

What can you say about friction? Do you know what it stands for? Friction acts when there is contact between two surfaces. Friction is the resistance between two surfaces that are in contact with each other. Friction is greater when an object is on a rough surface rather than on a smooth surface. How about frictional force? Do you know what it is?

Can you give some examples of friction? Let us look at Figure 3.1 for two examples of friction.

Frictional force is a force that opposes the direction of motion and acts in the opposite direction of the motion.

3.1

A force is a ppush or pull upon an object as a result of the object's interaction with another object.

� TOPIC 3 FORCE AND ENERGY 70

Figure 3.1: Examples of friction

Below are explanations for the examples in Figure 3.1: (a) If we skid a book across the surface of a desk, then the desk exerts a friction

force in the opposite direction of its motion. This stops the movement of the book after a while.

(b) When we want to slow a bicycle quickly, we need to have a lot of friction

between the brake blocks and the wheels as they touch.

3.1.2 Magnetic Force

I am sure you know what a magnet is. How about magnetic force?

A magnet also exerts force on another magnet. The force that exists between two magnets can be a force of attraction or repulsion. Figure 3.2 shows a simple experiment to study the force that magnets exert on each other.


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Figure 3.2: Two magnets exerting force on each other

3.1.3 Gravitational Force

Another type of force is gravitational force. Gravity holds objects in place on the EarthÊs surface. As we ascend from the EarthÊs surface, the pull of gravity decreases. It is gravity which causes all objects to have weight. Thus, the weight of an object is the force of gravity pulling that object down. Can you think of any example?

One example is when you kick a ball into the air; it will fall back to the ground. This shows that gravitational force has pulled the ball back to the EarthÊs surface.

3.1.4 Electrostatic Force

The last type of force for you to learn is electrostatic force. Let us look at its definition. The attractive and repulsive interaction between any two charged objects is called an eelectric force. One simple example of this force is the plastic comb. After running a plastic comb through your hair, you will find that the comb attracts bits of paper (see Figure 3.3).

Figure 3.3: Bits of papers get attracted to a comb


The attractive force is often strong enough to suspend the paper from the comb. Try this out. The same effect occurs with other rubbed materials, such as glass and hard rubber. When materials behave this way, they are said to have become electrically charged and the force is called eelectrostatic force.

Effects of Forces

As we know, we cannot see force. However, we can see and sometimes feel the effects of forces. Do you know that there are six effects of forces? Let us look at these effects as explained in Table 3.1.

3.2

Why does a parachutist fall more slowly to the ground when he uses a larger parachute? Explain.

ACTIVITY 3.1

For each situation listed, name the type of force at work.

(a) A magnet pulls a nail out of a box.

(b) A test tube, after being rubbed with wool, attracts small pieces of cork particles.

(c) A man pushes a stone up a hill.

(d) A ball stops rolling.

(e) A satellite is held in orbit above the Earth.

SELF-CHECK 3.1


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Table 3.1: Six Effects of Forces

Effect EExample

Move a stationary object

You can kick a ball to start off a football game.

Slow down or stop a moving object

A parachute can make an object slow down because of air resistance.

Change the speed of a moving object

When you hit a tennis ball, it speeds up.

Change the direction of a moving object

You can make a cricket ball change direction by hitting it with a bat.

Change the shape of an object

If you squeeze or kick a football, it will be compressed. This change may be permanent or temporary.

Change the size of an object

When you squeeze a sponge, its size changes.


Now, let us get to know interesting facts about force! Here they are:

FORCE FACTS!

(a) Measured in Newton (N).

(b) Usually acts in pairs.

(c) Acts in a particular direction.

(d) Usually cannot be seen but the effects can be felt.

Label the force in each picture as a push or pull. Then, describe whether the force is causing a change in speed or direction or both.

ACTIVITY 3.2

1. Define force. 2. Give an example when a force:

(a) Changes the shape of an object.

(b) Changes the direction of a moving object.

(c) Changes the speed of a moving object. 3. Explain four effects of forces.

SELF-CHECK 3.2


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3.2.1 Force Can Change Shape and Movement of Objects

Force can change the shape of an object. A simple activity to manifest this is by throwing a brick into the mud in a basin. The mud is splashed out from the basin due to the impact of the force from the brick. Another example is about various materials that require more or less pulling force so that the material can be torn apart. A tissue paper will need less pulling force while a cardboard will need more force to be torn apart. When force is exerted on a soft object, the object becomes squeezed, stretched, bent, twisted or squashed. When force is exerted on a fragile object, the object will be broken. For example, a glass bottle will disintegrate if it falls on the floor.

By understanding the nature of a force, people can prevent their belongings from colliding. For example, letÊs say a small boy is riding a bicycle. He avoids colliding with trees because if the bicycle collides, it will experience a change in shape (Figure 3.4).

Figure 3.4: Crushed bicycle

Source: http://www.wizardsofaz.com

It is the same if you throw plasticine onto a wall. The plasticine will also experience a change in shape. In order to understand the reason behind those two situations, think about when pushing certain objects, they will push back to you. So does the bicycle. When the bicycle hits the tree, the tree will „hit‰ back at the bicycle. As a consequence, the bicycle will experience some changes in shape (crushed). The same principle also applies to the plasticine. Once it hits the wall, the wall will „hit‰ back at the plasticine. As a result, the plasticine experiences change in shape.


When you play badminton, you will probably smash the shuttlecock to make it move faster so that you may defeat your opponent in the game. This activity shows that force can speed up the motion of an object. When you go on a shopping spree at the supermarket, you will use a trolley to bring the groceries around (Figure 3.5).

Figure 3.5: Pushing a trolley

Source: http://www.bbc.co.uk

Sometimes, you will stop the trolley at certain sections in the supermarket to observe things you are interested in buying. Then, you start to push again to move to other sections. When you hold the moving trolley, force is applied to the trolley to make it stop. When you push it back, force is applied to the trolley so that it starts moving. Sometimes, you push it harder so that it can move faster. By doing this activity, you are actually putting more force on the trolley.

Table 3.2 shows the different types of forces on objects.


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Table 3.2: Six Types of Forces

Force Explanation

Propulsion This can be any driving force, it may be a push or pull, but it could be an engine which forces an object forward

Water resistance

This is a force which acts in water; it can slow objects down, reducing the effect of gravity.

Air resistance

This is a force which acts in the air, it can slow objects down when they are moving against it or if harnessed could be used to move an object along.

Friction This force acts on objects when they are in contact with a surface, such as the ground. It can be reduced by ensuring both surfaces are smooth.

Gravity This force affects every object on Earth. It is a force which pulls everything to the centre of the Earth.

Upthrust This is an upward force which acts in water; it acts on an object against gravity and it is the reason why certain objects float.

3.2.2 Friction

Let us do this activity to learn more about friction:

It is all about friction. Friction is a force that opposes the movement of an object. In other words, the direction of a frictional force is always against its motion. For example, if the direction of a moving object is towards the left, the direction of the frictional force is towards the right.

Friction occurs when two surfaces rub against each other. For example, when a marble rolls on the floor, friction occurs when the surface of the marble rubs against the surface of the floor. Eventually, the marble will stop rolling because the frictional force longer that the force of the moving marble.

Effects of Friction Force on Movement of Object Let your students do the following activities to experience and observe the effects of friction on the movement of an object.

(a) Rub your palms together. What do you feel? Do you feel warm? You can do this when you feel cold.

Roll a marble on two types of surfaces: smooth and rough. Can you predict which marble will roll a longer distance? Can you explain why a marble rolls a longer distance on a smooth surface than a rough surface?


(b) Got an eraser? Rub it on a piece of paper as much as you can. Observe what happens to the size and shape of the eraser. Make a conclusion based on this observation. Relate it to the concept of friction.

(c) Ride a bicycle on a smooth road and then on a field full of grass. Which place needs more energy for you to ride the bicycle?

The condition of the surfaces that rub against each other is a factor that contributes to friction. A small friction occurs when the surfaces are smooth whereas there will be greater friction when the surfaces are rough. This is why it is harder to ride a bicycle in a field full of grass than a smooth road.

The type of surface also affects the distance moved by the object. If a child rolls a toy car on different surfaces, the car will move the furthest on the smoothest surface and the slowest on the roughest surface. At home, have you ever noticed how easy it is to move a small table compared with a big refrigerator? The reason is friction also depends on the weight of an object. A heavier object exerts a greater frictional force.

Students should be taught about friction as a force that slows moving objects and may prevent objects from starting to move. For that, you may discuss the following: (a) Air resistance: Investigating parachutes.

(b) Water resistance: Why is it hard work to walk through water?

(c) Why are fish the shape they are?

(d) Why are boats and ships the shape they are?

(e) Investigating different surfaces: Smooth and rough.

Students also should be taught that when objects are pushed or pulled, an opposing pull or push can be felt. For that, you may do and discuss the following exercise: � Practical: Stretching a spring balance. When they pull harder that is

using a bigger force, the spring balance records a higher reading in Newton.

ACTIVITY 3.3


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Remember that friction is both an advantage and a disadvantage. Friction allows us to walk without falling over and to pull a thread with a needle. Smooth car tyres have less friction on the road than tyres with tread. Which tyre is safer? Friction is also the cause of wear of the moving parts of engines so you have to reduce friction with lubricants such as motor oil. The simplest method of reducing friction is to place rollers between the two surfaces. This method is used when boats are launched, or when heavy wooden crates have to be moved. Here linear friction is replaced by rolling friction. Ball bearings are used to reduce friction for revolving shafts. The axle of a bicycle is mounted in ball bearings. The ball race is similar to the ball race used in a bicycle. Friction is thus reduced by: (a) Replacing linear friction with rolling friction; and

(b) By using hard surfaces, as hard surfaces have less friction than softer surfaces.

Lubrication is the most common method of reducing friction. On a bicycle all moving parts have small holes through which oil is squirted so that the parts that move in contact with one another are covered in oil. How does friction help in increasing or decreasing the speeds of athletes at sports events?


Fill in the crossword puzzle.

7 4 1 2 6 5

9 8 3

Hints:

1. Friction allows us to walk or run without _ __________ .

2. Friction enables us to hold things because it __________ the object from moving.

3. The brake system in vehicles makes use of friction to ________ the vehicles.

4. If there is no friction, you will not be able to _______ properly on the ground.

5. Friction enables us to ______ a knife and other instruments.

6. Friction produces _______ which can damage some parts of machines.

7. Friction causes surfaces which are rubbing ________ each other to wear out.

8. Friction also causes wasting of ___________.

9. Worn-out tyres are ________ because they can slide and skid easily, causing accidents to occur.

SELF-CHECK 3.3


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3.2.3 Applications of Frictional Force

Before we go to the applications of frictional force, let us recall what it is. Can you give the meaning of frictional force? Frictional forces are present everywhere in our daily life. Friction is useful in many instances. Without friction, we would not be able to walk or run and cars will not move. Friction also prevents our feet from slipping. It is simply impossible to reduce frictional forces completely. Frictional forces are useful in some situations but can be an obstruction in others. As stated in its definition, frictional forces exist between surfaces of two objects being in contact. Their directions are always parallel to that surface and opposite of the direction of the intended motion of an object (see Figure 3.6).

Figure 3.6: Frictional force

There are many ways to reduce friction. Find out how to reduce friction using:

(a) Aerodynamic shape.

(b) Rollers or ball bearings.

(c) Lubricants such as oil, wax or grease.

(d) Talcum powder or air cushion.

(e) Smoothening surfaces in contact.

ACTIVITY 3.4


Let us turn on our time machine to discover the early applications of this force. The first practical application of friction occurred nearly a million years ago, when it was discovered that heat from friction could be used to light a fire. The use of liquid lubricants to minimise the work needed to transport heavy objects was discovered more than 4,000 years ago. IsnÊt that interesting? However, friction has its disadvantages. Friction opposes motion. Thus, you have to add more energy to move an object. This reduces efficiency and leads to wastage of energy. Friction also causes wear and tear of moving parts in machines such as cars. That is why we need to replace certain parts of our cars after some time. When you use a saw to cut up something, be careful with its blade. It tends to heat up when a lot of sawing is done. This causes the metal to snap when it becomes very hot. This sounds scary, doesnÊt it? But with proper equipment and precautions, I am sure this kind of incident can be avoided. Let us try a simple activity.

By the end of this experiment, can you describe the characteristics of materials which provide good grip? Is it rubbery, soft, grippy, bendy, smooth or rough? What conclusion can you make of this experiment? Well, next time you want to buy a new pair of shoes, you might want to think about having one with increased frictional forces rather than grip! How about the objects in or on water? Do you know that water resistance or friction does exist? Well, being in water, ships, boats, submarines and fish all have to face the similar effects of water resistance or friction. Let us take a fish as an example. Have you ever thought how fish reduce the frictional forces on their bodies as they move through water? The answer is having a streamlined shape, slippery surface and few protrusions (see Figure 3.7)!

Experiment 3.1

Objective: To identify the characteristics of materials with good grip.

Procedure:

(a) Place different items of footwear on a tray or other flat surface.

(b) Carefully tip the tray. You will notice that the shoes with more grip (greater frictional forces) will be the last to move. Try this with different shoes. Predict which will be the best.


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Figure 3.7: A fish has a streamlined shape, slippery surface and

few protrusions to reduce frictional force in water So, how does a submarine copy the fishÊs characteristics in order to face the effects of water resistance? By studying the characteristics of a ship, we humans do the best we can in designing our submarines, ships and boats. This is done by choosing shapes that will glide through the water and materials which are as smooth as we can economically make them. As mentioned before, frictional force can be our friend and also our enemy. One simple example of being our friend is that friction helps us to walk (Figure 3.8).

Figure 3.8: Friction helps us to walk

Source: http://www.csicop.org Have you ever tried to walk on ice? Try walking around the room you are in. If you walk slowly and concentrate hard you may be able to feel each foot as it pushes down and slightly backwards with each step. Without this friction allowing you to push back against the floor, you would get nowhere.


How about Olympic swimmers? Olympic swimmers reduce the frictional forces of water resistance by using these techniques: (a) Smooth swimsuits;

(b) Bald heads;

(c) Swimming caps; and

(d) Removal of body hair. Interesting isnÊt it? How about the aeroplane? How do aeroplane manufacturers reduce the frictional forces of air resistance in their planes? The planes are made with: (a) Smooth materials;

(b) Aerodynamic shape; and

(c) Few protrusions. Let us do a thought experiment. Imagine you are trying to drive away from a muddy field. But you are having difficulty because the wheels of your car are spinning on the mud. This is happening because there is not enough friction between your tyres and the ground. The mud is acting as a lubricant (rather like oil) and reducing the friction. So, how could you get out of this sticky mess? You need to increase the frictional force to make the car move. How? You could put sand or stones beneath the wheels to reduce the lubricating effect of the mud. The wheel could „get a grip‰ and you can get drive away in no time! Can you think of some daily activities that can be associated with friction? Check out the answers in Figure 3.9.


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Figure 3.9: Friction in daily activities

Find and observe one way that friction affects you in your daily life. After you have chosen one friction event from your daily life, answer the following questions and combine them into one paragraph. (a) What are the two things which rub each other?

(b) How is friction helpful or hurtful in this case?

(c) If the friction is harmful, how can you reduce it? If it is helpful, what can be done to increase the friction?

(d) Can you think of a similar case in which friction will have the same effect?

ACTIVITY 3.5


MEASUREMENT OF FORCE

In this subtopic, we will learn about unit of forces and the principle of spring balance.

3.3.1 Unit of Forces

As with all physical quantities, force also has a method of measurement. Force has a certain magnitude or size and acts in one direction. We measure force in Newton. How do we measure this? We measure force by using an instrument called a spring balance or Newton balance (Figure 3.10). The spring scale can also be called a Newton meter because it is used to measure Newton (Figure 3.11).

Figure 3.10: Spring balance

Figure 3.11: Different types of spring balance

3.3


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As mentioned earlier, a force is a vector quantity while force is a quantity which is measured using the standard metric unit known as Newton. A Newton is abbreviated as "N." How about one Newton (N) of force? One Newton (N) of force is defined as the amount of force needed to accelerate one kilogram (kg) of mass at a rate of one metre per second squared (ms-2). In simpler form:

1 Newton = 1 kgm/sec2

Force = Mass times acceleration (F = ma) Where m = mass and a = acceleration What do you think happens to a force when the weight of object acting on the spring is increased? Yes, the magnitude of the force will be increased. The force of gravity acting on an object is the weight. We can measure the gravitational force acting on an object by using a spring balance. Look at the example in Figure 3.12 which shows a spring that is extended by a load. Example 3.1: Let us calculate the value of P (in centimetres) when a load of 140 g is attached to a spring balance (Figure 3.12).

Figure 3.12: Spring balance extended by a load


Solution: From the diagram, we can see that when 40g of load is attached to the hook, the spring extends by 2cm. Since force is proportionate to the mass of the load:

140P � ��

2= 7cm

40

We learnt earlier that there are four types of forces. Can you recall them? These are magnetic, gravitational, electrostatic and frictional. Now, let us look at the magnitude of frictional force. Do you think different types of surfaces affect the magnitude of frictional force? I am sure most of us will say yes to the question. Let us verify this statement by doing an experiment. Let us say that our hypothesis is that different types of surfaces have different magnitudes of frictional force. We also hypothesise that the rougher the surface, the greater the frictional force. We are going to use four different types of surfaces. We call this our manipulated variables. Our constant variables will be the mass of the wooden block used. The force exerted on the wooden block will be the same. We will then observe the magnitude of the frictional force on each type of surface used. Let us begin the experiment to prove our hypotheses.


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The area of contact does not affect the magnitude of the frictional force. Thus, we can conclude that the magnitude of frictional force depends on the nature of the contact surfaces.

Experiment 3.2

Objective: To verify whether different types of surfaces affect the magnitude of frictional force. Procedure:

(a) Place a wooden block with its broader surface resting on a table.

(b) Attach the block to the spring balance, just like in the Figure 3.13 below.

Figure 3.13: Block attached to the spring balance

(c) Pull the spring balance horizontally across the table until the wooden block begins to move.

(d) Record the reading of the spring balance when the block begins to move.

(e) Repeat the procedure by pulling:

(i) The broader surface of the wooden block on a piece of sandpaper.

(ii) The broader surface of the wooden block on a smooth sheet of glass.

(iii) The narrow surface of the wooden block across the table. Results: Your results would show that the rougher the surface, the greater the frictional force between them.


Are you aware that there are many natural phenomena occurring around us? They are closely related to the force of gravity. Why does an object fall to the ground if you drop it from a tall building? When you throw an object into the sky, why does it fall back to the Earth?

3.3.2 Principle of Spring Balance

Before we look at the principle of spring balance, let us look at its definition first. The spring scale apparatus is simply a spring fixed at one end with a hook attached to an object at the other (Figure 3.14).

Figure 3.14: A spring scale apparatus

Do you know how a spring balance works? It works by following HookeÊs Law, which states that the force needed to extend a spring is proportional to the distance that spring is extended from its rest position. This balance measures weight or force by how far a spring moves. The scale is designed to read weights correctly when they are hanging straight down.


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Let us look at how a spring balance works. Look at the pointer on the spring balance in the previous Figure 3.14. It gives the reading of the magnitude or force. When you attach an object to the hook, it will pull the spring and stretch it. Now, the extension of the spring depends on the weight of the load or object you attached to the spring. The weight of the object exerts a force on the spring. The greater the force is, the more the spring will stretch. Therefore, the weight of the object is the force of gravity acting on that object. In physics, a force is a quantity that produces a change in the size, shape or motion of a body. Commonly called a „push‰ or „pull,‰ force is a vector quantity. What is meant by vector quantity?

To fully describe the force acting upon an object, you must describe both the magnitude (size or numerical value) and the direction. Thus, 10 Newton is not a full description of the force acting upon an object. In contrast, 10 Newton downwards is a complete description of the force acting upon an object; both the magnitude (10 Newton) and the direction (downwards) are given. However, you must bear in mind a factor which influences the use of the spring balance. The spring balance must be in a good state of repair to function correctly. Figure 3.15 shows you an example of how to use a spring balance to measure force in a lab.


Figure 3.15: Using a spring balance to measure force

GRAVITY

All objects are held to the surface of the Earth due to the force of gravity. Without it, the spinning of the Earth will send the objects floating off into outer space. Gravitational pull causes objects to be pulled towards the centre of the Earth. The gravitational attraction of the moon and the sun on the Earth and its oceans also cause the ocean tides.

3.4

Let us use the spring balance. Follow these steps:

(a) Set up the apparatus as in Figure 3.15.

(b) Record the initial reading of the spring balance.

(c) Attach a 500 g weight on the spring balance.

(d) Record the reading of the spring balance.

(e) Repeat the process by replacing the weight with a book, a stopwatch, a beaker, a measuring cylinder and a ruler.

(f) Record the readings.

(g) Write down your conclusion.

ACTIVITY 3.6


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In general, gravitation is a force that causes objects to pull them towards one another. The term gravitation refers to force in general, while the term gravity refers to the Earth's gravitational pull. Although this section only deals with gravitational force from Earth, we should be aware that gravitation can also be found on other planets in our solar system including the sun. Gravitational force keeps the Earth and other planets of the solar system in orbit around the sun. The Earth, as a planet, also has its own gravitational force that keeps the moon in orbit around the Earth.

When an object falls freely towards the Earth, there is constant acceleration. The speed of the object falling freely near the EarthÊs surface increases by about 9.8ms-2. Although the value of gravitational acceleration is generally about 9.8ms-2, the g values can more or less vary in certain places around the world. Table 3.3 shows you some examples of different gravitational accelerations in several cities around the world.

TRAVELLING INTO OUTER SPACE

Travelling in outer space is the most critical obstacle. This is due to the presence of the gravitation of the Earth. A vehicle that is supposed to travel into outer space must be able not only to transport but also to move away from the Earth; that is against the force of gravity.

WHICH PLACE HAS A STRONGER STRENGTH OF GRAVITY?

Depending on the location, the strength of gravity is not the same at all points on the Earth. The matter that determines the strength of gravity at any given place is the distance from the centre of the Earth. Consider the distance of an object from the centre of the Earth. Any object, for example, a house at the seashore, is at a lower elevation than one in the mountains or higher elevation. This means that it is closer to the centre of the Earth. Therefore, the strength of the gravity is stronger at the seaside house than at the mountain house.


Table 3.3: The Gravitational Acceleration in Several Cities

City GGravitational Acceleration (ms-2) Auckland 9.799

Buenos Aires 9.797 Chicago 9.803 Istanbul 9.808 Helsinki 9.819 Kuwait 9.793 Lisbon 9.801

Mexico City 9.779 Paris 9.809 Rome 9.803

Sydney 9.797 Tokyo 9.798

Early Idea about Gravity

In the mid-16th century, a Polish astronomer, Nicolaus Copernicus, had proposed a heliocentric or sun-centred system. He believed that the planets moved in circles around the sun. At the beginning of the 17th century, the Italian physicist and astronomer, Galileo Galilei, agreed with this cosmology theory proposed by Copernicus.

However, Galileo believed that the planets moved in circles because this motion was the natural path of a body with no forces acting on it. He saw no connection between the force behind planetary motion and gravitation on the Earth. Galileo also theorised that all objects fall towards the Earth with the same acceleration when gravity is the only force acting on them, regardless of their weight, size or shape.

In the late 16th and early 17th centuries, based on observations without telescope, Danish astronomer Tycho Brahe and his student, the German astronomer Johannes Kepler, found that planets did not move in circles but in ellipse. In 1609, using a telescope, Galileo observed that the moons orbited the planet Jupiter, a fact that could not reasonably fit into an Earth-centred model of the heavens.

In the late 17th century, Isaac Newton developed a theory of gravitation that encompassed the attraction of objects on the earth and planetary motion.


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If the Earth is Round like a Ball, Why Aren't We Standing at Odd Angles Around it?

Well, the fact is, we are all standing at odd angles around the Earth. It just does not feel like it because of the force called gravity. You are pulling on the Earth just as hard as it is pulling on you!

However, since your weight is much less than the Earth, you do not affect its motion at all. However, the Earth has a big pull on you. This pull is what keeps all of us from flying off into space. Every object pulls towards its centre. No matter where we are, when we are standing, the force of gravity points away from our feet into the Earth.

So, if you visit someone on the opposite side of the world, the gravitational force on you will be in the opposite direction of that which is on your parents, who are still at home. So, we really are standing at odd angles. When astronauts go into outer space, they can float because they feel less of a pull from Earth.

The marble and the feather

The weight of an object is affected by the pull of the EarthÊs gravity on the object. In other words, the weight of an object is equal to the mass of the object multiplied by the acceleration due to EarthÊs gravity. All objects which are in freely falling condition will accelerate in the same manner regardless of their weight, size or shape. When we apply this principle in real life, it is predicted that a marble and a feather will both fall at 9.8ms-2 when the pull of gravity is the only force acting on them. However, this principle is difficult to be applied or observed in daily life. It has been found that a feather falls more slowly than a marble. Discuss and find out more about this unique phenomenon.

ACTIVITY 3.7


WORK, POWER, ENERGY AND EFFICIENCY

Now we come to the subtopic which focuses on work, power, energy and efficiency.

3.5.1 Work

The word work has a variety of meaning in everyday language. In everyday life, it refers to any kind of mental or physical activities. But in physics, work is described as what is accomplished by the action of a force when it acts on an object when the object moves through a distance. Specifically, the work done on an object by a constant force (in both magnitude and direction) is defined as the product of the magnitude of the displacement times the component of force parallel to the displacement, as shown in Figure 3.16.

Figure 3.16: Force that produces work

W = F x d where W = work done F = force parallel to the displacement d = displacement

The means that no matter how big the force exerted on the object, there will be no work done if the object does not move! Consider the case when the motion and the force are not in the same direction (Figure 3.17).

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Figure 3.17: Examples where no work is done

You will have to find the component of force which is parallel to the direction of motion. Using trigonometry, the force parallel to the direction of motion is F cos �, so the work done now is

W = F cos � x d or W = Fd cos �

If a man is pulling a box along the floor at an angle of 38À, the box will slide on floor and the direction of movement is parallel to the floor (Figure 3.18).

Figure 3.18: A man pulling a box along a floor


If we have to calculate the work done by the man pulling the box, we have to find the component of force which is parallel to the floor, which is 450 cos 38À. For the case of the man carrying a heavy box (previous Figure 3.17b), the direction of force is perpendicular (90À) to the direction of movement, so the work done is F d cos 90À = 0. So the work done is zero even though the man is carrying a very heavy load.

3.5.2 Unit and Calculation of Work

Now we come to the unit and calculation of work.

1 joule (J) = 1 Newton (N) x 1 metre (m) = 1 Newton metre (Nm)

In other words,

Let us look at some examples. Example 3.2: A father lifted up his son weighing 300 N to a vertical height of one metre. How much work is done on the son? Solution: Work done = Force Distance = 300N 1m = 300J

The SI unit for work is joule (J) or Newton metre (Nm)


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Example 3.3: A Science teacher used up 300J of energy to push a trolley weighing 150N along the corridor. Calculate the distance over which the trolley is moved. Solution: Force Distance = Work done

Distance = Work done 300J

= = 2mForce 150N

Example 3.4: A labourer does 1,500J of work when he pushes plough over a distance of six metres. How much force does the labourer use? Solution: Force Distance = Work done

Force = Work done 150J

= = 250NDistance 6m

Example 3.5: A student drags a 60kg bag with a force of 50N along a rope which makes an angle of 40� with the horizontal. The bag moves a distance of 3m. Find the work done by the student. Solution: Component of force in the direction of motion, F = 50 cos 40À = 50 0.766 = 38.3N Work done, W = 38.3N 3m = 114.9J


3.5.3 Power

Now, let us learn about power. Do you know what power stands for? When an engine does work quickly, it is said to be operating at a high power. If it does work slowly it is said to be operating at low power.

1. A spring balance is hooked to a 600g wooden block. It is pulled at a constant speed over a distance of two metres. The reading of the spring balance and the distance are recorded. Next, it was dragged over a distance of three metres at the same speed. The readings were also recorded. A 1,200g wooden block is then dragged over a distance of two metres. The reading on the spring balance is recorded. Based on the readings recorded, calculate the work done and fill in the answers in the table below.

Activities Force Used (N)

Distance Moved (m) Work Done

600g block dragged over 2m 3 2

600g block dragged over 3m 3 3

1,200g block dragged over 2m 6 2

2. Indicate whether the following statements are TRUE or FALSE.

(a) More work is done when the 600g wooden block is dragged over a distance of 3m compared with a distance of 2m.

(b) Less work is done when the 1,200g wooden block is dragged over a distance of 2m when compared with the 600g weight being dragged over the same distance.

(c) The greater the distance over which a load is moved, the greater the amount of work.

(d) The heavier the load, the lesser the amount of work done.

SELF-CHECK 3.4


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3.5.4 Unit and Calculation of Power

Let us look at a simple example to better understand power. Your brother, who is bigger than you, might carry a box up a flight of stairs in 20 seconds. However, you, being of smaller size than him, took 40 minutes to carry the same box up the same flight of stairs. Both of you have done the same amount of work. However, your brother did the work faster than you. Thus, he has used up more power than you to get the work done. The equation for the calculation of power is as follows:

From the equation, we can see that the faster the work is done, the greater is the power. Power is measured in watts (W) or joules per second (Js-1).

One watt is one joule of work done in one second

1W = 1Js-1

Power (watts) = � ��

Work done joules

Time taken seconds

Power is defined as rate of energy transfer (work done per second) or energy spent per second.


Let us look at some examples. Example 3.6: How much power is required by an engine to raise a 400N box to a height of 20m in two minutes? Solution:

� � � ��

Work done joulesPower watts =

Time taken seconds

Total force Distance=

Time taken

400N 20m=

2 60s

= 66.67W

Let us check out who has the most power by following these steps: (a) Everyone should do this activity. Get your weight using the

weighing machine.

(b) Measure the vertical height of the stairs.

(c) Each student must walk quickly to the top of the stairs.

(d) Record the time taken for each student to reach the top of the stairs.

(e) Calculate the power of each student. Who generates the most power to reach the top of the stairs? Does speed affect power? Explain.

ACTIVITY 3.8


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Example 3.7: A student who weighs 500N runs up a flight of stairs which is seven metres high in 50 seconds. How much power does the student use to run up to the top of the stairs? Solution:

� � � ��


Time taken seconds

Total force Distance=

Time taken

500N 7m=

50

= 70W

Example 3.8: Calculate the time taken by a machine which uses 5,000W to lift a load weighing 2,000N to a vertical height of five metres. Solution:

� � � ��


Time taken seconds

Work doneTime =

Power

2000N 5m=

5000W

= 2s


3.5.5 Potential Energy and Kinetic Energy The energy of an object due to its motion is defined as kkinetic energy. All moving objects have kinetic energy. The faster an object moves, the more kinetic energy it has. Can you think of some examples of objects that possess kinetic energy?

Suppose the object of mass m is moving with a velocity v. The kkinetic energy of the object, Ek is

Ek = � mv2

Did you know that an object has energy even though it is not moving? We call this ppotential energy. Potential energy is the energy stored in an object because of its position or its condition.

Potential energy is the energy of an object because of its higher position in the gravitational field. Because of this, potential energy is also known as gravitational potential energy. The higher the object is raised above the ground, h the more gravitational potential energy it has.

The formula for gravitational potential energy is given by

Ep = mgh

1. Define work. Name the two components of work.

2. State the units used for measuring force, distance and work done.

3. Give three examples of situations where work is done and three examples of situations where work is not done.

4. A box of books with a mass of 10kg is lifted from the floor onto a table at a height of 0.4m. Calculate the amount of work done.

5. Define power. State the units for energy, work and power.

6. A boy who weighs 450N can run up a flight of stairs of 20 steps, in five seconds. Given that each step is 20cm high, how much power does the boy use to run up the flight of stairs?

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If a mass of an object is m= 5kg, and the object is held stationary at a height h=4 m, the gravitational potential energy, Ep

Ep = (5kg) (9.8 m s-2) (4m) = 196J

3.5.6 Energy Conservation

When any object falls in the earthÊs gravitational field, a small part of the energy is used up to overcome the air resistance. This energy is dissipated or lost as heat. Although energy may be transferred from one form to another, the total energy in the system is always the same.

An example is an electric motor of a fan. If the electric motor is supplied with 100J of energy, maybe only 850J is transformed into mechanical energy. The remaining 140J may have been transformed into heat energy and another 10J into sound energy.

A simple example of the conservation of mechanical energy is a ball allowed to fall from a height h as shown in Figure 3.19.

Figure 3.19: Object falls from rest

The principle of conservation of energy states that energy ccannot be created or destroyed but can cchange from one form to another. The total energy is thereby cconstant.


The instant it is dropped, it has maximum potential energy of mgh and as it falls, the potential energy decreases as h decreases, but the kinetic energy increases to compensate for the loss of energy. Thus the sum of two energies remains the same. Let us carry out an activity. Rub your palms together. What do you feel? I am sure your palms feel warm. In this exercise, kinetic energy has converted to heat energy. How about when you light a candle and wait for it to burn? What energy changes have taken place?

3.5.7 Sources of Energy

There are various forms of energy. These forms of energy can come from sources such as nuclear radioactive form, hot objects, vibrations of an object and many more. Let us talk about specific examples. Heat energy is the energy released by a hot object, chemical energy stored in living cells is converted to electrical energy, elastic potential energy is the energy stored in an elastic object like the spring of a motorcycle and sound energy can be the energy released by the sound of vibrating objects.

3.5.8 Simple and Compound Machines

A machine is a tool that helps us to do work. It can change the amount or direction of a force. They are considered simple because they used the simplest mechanism to provide this change of leverage of the force. Technically there are six classical simple machines that were defined by scientists during the renaissance period. These machines include the inclined plane, lever, wedge, wheel and axle, screw and pulley. The simple machine will contain devices and mechanisms to cchange the force. These simple machines are used to create more complicated machines and most machines can be decomposed and separated into any of these six simple machines. (a) LLever A lever is a simple machine. A lever is a board or bar that rests on a turning

point. This turning point is called the fulcrum. An object that a lever moves is called the load. The closer the object is to the fulcrum, the easier it is to move. A hammer is a lever when it is used to pull a nail out of a piece of wood. Examples of lever are:

(i) Bottle openers; and

(ii) Crow bar.


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Figure 3.20 shows three types of levers, which are the first class lever, second class lever and third class lever. Observe the positions of the load, effort force and fulcrum in each of the cases.

Figure 3.20: Different classes of levers

(b) Inclined Plane An inclined plane is a simple machine. It is a flat surface that is higher on one

end. You can use this machine to move an object to a lower or higher place. Inclined planes make the work of moving things easier. You would need less energy and force to move objects with an inclined plane. Examples of inclined planes:

(i) Ramp;

(ii) Slanted road;

(iii) Path up a hill; and

(iv) Slide. (c) Wheel and Axle The wheel and axle is another simple machine. The axle is a rod that goes

through the wheel. This lets the wheel turn. It is easy to move things from place to place with wheels and axles.


Examples of wheels and axles:

(i) Cars;

(ii) Roller skates;

(iii) Wagons;

(iv) Door knobs; and

(v) Gears in watches, clocks, and bicycles. (d) Screw A screw is a simple machine that is made from another simple machine. It

is actually an inclined plane that winds around itself. A screw has ridges and is not smooth like a nail. Some screws are used to lower and raise things. They are also used to hold objects together.

Examples of screws:

(i) Jar lids;

(ii) Light bulbs;

(iii) Stools;

(iv) Clamps;

(v) Jacks;

(vi) Wrenches;

(vii) Key rings; and

(viii) Spiral staircase. (e) WWedge A wedge is a simple machine used to push two objects apart. A wedge is

made up of two inclined planes. These planes meet and form a sharp edge. This edge can split things apart. Examples of wedges:

(i) Knives;

(ii) Axes;

(iii) Forks; and

(iv) Nails.


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(f) Pulley A pulley works by changing the direction of force, or the total force acting

on a system. There are two types of pulley, which are the stationary pulley or the movable pulley. A pulley system is made up of a combination of stationary of movable pulleys.

This simple machine is made up of a wheel and a rope. The rope fits on the

groove of the wheel. One part of the rope is attached to the load. When you pull on one side of the pulley, the wheel turns and the load will move. Pulleys let you move loads up, down, or sideways. Pulleys are good for moving objects too hard to reach places. It also makes the work of moving heavy loads a lot easier.

Examples of where pulleys can be used:

(i) Flag poles;

(ii) Clothes lines;

(iii) Sailboat;

(iv) Blinds; and

(v) Crane. A compound machine is a machine which is made up of two or more

simple machine. In Figure 3.21, identify the simple machines that made up the compound machines.

Figure 3.21: Examples of compound machines


PRACTICAL INVESTIGATIONS IN THE PRIMARY SCIENCE CURRICULUM

In the primary science curriculum, pupils studied force and motion (Year 6), by doing simple activities like pushing, pulling, wringing, blowing and crushing to study the effects of force on objects. To study the distance travelled by different objects at different speeds at a given time, pupils experiment by releasing trolleys at different heights and measure the distance travelled at a given time. Pupils also conduct experiments to investigate the movement of objects on different types of surfaces; smooth or rough surfaces like glass, cardboard and sandpaper.

� Force has a certain magnitude or size and acts in one direction. We measure

force in Newton.

� There are many types of forces around us. Among them are frictional, magnetic, gravitational and electrostatic.

� There are six effects of force. Among them are to move a stationary object, slow down or stop a moving object, change the speed of a moving object and change the shape of an object.

� A spring balance is used to measure force. It is a spring fixed at one end with a hook attached to an object at the other. It works by HookeÊs Law, which states that the force needed to extend a spring is proportional to the distance that spring is extended from its rest position. The weight of an object on a spring balance is the force of gravity acting on that object.

� Force is a vector quantity. A vector quantity is a quantity which has both magnitude and direction.

� The magnitude (size or numerical value) and the direction describe the force acting upon an object.

� All objects are held to the surface of the Earth due to the force of gravity. Gravitational pull causes objects to be pulled towards the centre of the Earth.

� Work is done on an object when a force is applied to move the object in the same direction as the direction of the force.

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� Work has two components:

� Force is applied on an object.

� The object moves a certain distance.

� Work equation is work (Joules) = Force (Newton) � Distance (Metre). The SI unit for work is joule (J) or Newton metre (Nm).

� Power is the rate of change of energy. The unit for power is Watt (W).

� Kinetic energy is defined as the energy of an object due to its motion. All moving objects have kinetic energy.

� Potential energy is the energy stored in an object because of its position or its condition.

� The principle of conservation of energy states that energy cannot be created or destroyed but can change from one form to another. The total energy is thereby constant.

� Machines make it easier for us to do work by changing the direction of the force.

� Examples of simple machines are lever, inclined plane, lever, wedge, wheel and axle, screw and pulley.

� A compound machine is made up of two or more simple machines.

Compound machine

Conservation of energy

Fiction

Force

Gravitational force

Lever

Power

Pulley

Screw

Simple machine

Spring balance

Wedge

Wheel and axle

Work


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Topic 3 Force and Energy

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Transcript of Topic 3 Force and Energy