Saturday, August 23, 2008Friction And Its TypesFrictionis theforceresisting the relative motion of twosurfacesin contact or a surface in contact with a fluid (e.g. air on an aircraft or water in a pipe). It is not afundamental force, as it is derived fromelectromagnetic forcesbetweenatomsandelectrons, and so cannot be calculated fromfirst principles, but instead must be found empirically. When contacting surfaces move relative to each other, the friction between the two objects convertskinetic energyintothermal energy, orheat. Friction between solid objects is often referred to as dry friction orsliding frictionand between a solid and a gas or liquid as fluid friction. Both of these types of friction are called kinetic friction. Contrary to many popular explanations,sliding frictionis caused not by surface roughness but by chemical bonding between the surfaces.[1]Surface roughness and contactarea, however, do affect sliding friction for micro- and nano-scale objects where surface area forces dominate inertial forces.[2]Internal friction is the motion-resistingforcebetween the surfaces of the particles making up the substance. Friction should not be confused withtraction. Surface area does not affect friction significantly, but intractionit is essential.

What is Friction?Friction is the "evil" of all motion. No matter which direction something moves in, friction pulls it the other way. Move something left, friction pulls right. Move something up, friction pulls down. It appears as if nature has given us friction to stop us from moving anything.Friction is actually a force that appears whenever two things rub against each other. Although two objects might look smooth, microscopically, they're very rough and jagged, as this picture shows:

As they slide against each other, their contact is anything BUT smooth. They both kind of grind anddragagainst each other. This is where friction comes from.But friction is not all bad. In fact, it has a lot to do with life as we know it here on Earth. Without it, we wouldn't be able to walk, sit in a chair, climb stairs, or use a mouse to surf the web. Everything would just keep slipping and falling all over the place.

Coulomb frictionCoulomb friction, named afterCharles-Augustin de Coulomb, is a model to describe friction forces. It is described by the equation:Ff = FnwhereFf is either the force exerted by friction, or, in the case of equality, the maximum possible magnitude of this force. is thecoefficient of friction, which is an empirical property of the contacting materials,Fn is thenormal forceexerted between the surfacesFor surfaces at rest relative to each other = s, where s is thecoefficientof static friction. This is usually larger than its kinetic counterpart. The Coulomb friction may take any value from zero up to Ff, and the direction of the frictional force against a surface is opposite to the motion that surface would experience in the absence of friction. Thus, in the static case, the frictional force is exactly what it must be in order to prevent motion between the surfaces; it balances the net force tending to cause such motion. In this case, rather than providing an estimate of the actual frictional force, the Coulomb approximation provides a threshold value for this force, above which motion would commence.For surfaces in relative motion = k, where k is the coefficient of kinetic friction. The Coulomb friction is equal to Ff, and the frictional force on each surface is exerted in the direction opposite to its motion relative to the other surface.This approximation mathematically follows from the assumptions that surfaces are in atomically close contact only over a small fraction of their overall area, that thiscontact areais proportional to thenormal force(untilsaturation, which takes place when all area is in atomic contact), and that frictional force is proportional to the appliednormal force, independently of the contact area (you can see the experiments on friction from Leonardo Da Vinci). Such reasoning aside, however, the approximation is fundamentally an empirical construction. It is a rule of thumb describing the approximate outcome of an extremely complicated physical interaction. The strength of the approximation is its simplicity and versatility though in general the relationship between normal force and frictional force is not exactly linear (and so the frictional force is not entirely independent of the contact area of the surfaces), the Coulomb approximation is an adequate representation of friction for the analysis of many physical systems.

Coefficient of frictionThe coefficient of friction (also known as the frictional coefficient) is a dimensionlessscalarvalue which describes the ratio of theforceof friction between two bodies and the force pressing them together. The coefficient of friction depends on the materials used; for example, ice on steel has a low coefficient of friction (the two materials slide past each other easily), while rubber on pavement has a high coefficient of friction (the materials do not slide past each other easily). Coefficients of friction range from near zero to greater than one under good conditions, a tire on concrete may have a coefficient of friction of 1.7.When the surfaces are conjoined, Coulomb friction becomes a very poor approximation (for example,Scotch taperesists sliding even when there is no normal force, or a negative normal force). In this case, the frictional force may depend strongly on the area of contact. Somedrag racingtires are adhesive in this way.The force of friction is always exerted in a direction that opposes movement (for kinetic friction) or potential movement (forstatic friction) between the two surfaces. For example, acurlingstone sliding along the ice experiences a kinetic force slowing it down. For an example of potential movement, the drive wheels of an accelerating car experience a frictional force pointing forward; if they did not, the wheels would spin, and the rubber would slide backwards along the pavement. Note that it is not the direction of movement of the vehicle they oppose, it is the direction of (potential) sliding between tire and road.The coefficient of friction is anempiricalmeasurement it has to be measuredexperimentally, and cannot be found through calculations. Rougher surfaces tend to have higher effective values. Most dry materials in combination have friction coefficient values between 0.3 and 0.6. Values outside this range are rarer, butTeflon, for example, can have a coefficient as low as 0.04. A value of zero would mean no friction at all, an elusive property evenMagnetic levitationvehicleshavedrag. Rubber in contact with other surfaces can yield friction coefficients from 1.0 to 2.

Static frictionStatic friction is a force between two objects that are not moving relative to each other. For example, static friction can prevent an object from sliding down a sloped surface. The coefficient of static friction, typically denoted as s, is usually higher than the coefficient of kinetic friction. The initial force to get an object moving is often dominated by static friction.Another important example of static friction is the force that prevents a car wheel from slipping as it rolls on the ground. Even though the wheel is in motion, the patch of the tire in contact with the ground is stationary relative to the ground, so it is static rather than kinetic friction.The maximum value of static friction, when motion is impending, is sometimes referred to as limiting friction,[3]although this term is not used universally.[4]The value is given by the product of the normal force and coefficient of static friction.Static frictional forces from the interlocking of the irregularities of two surfaces will increase to prevent any relative motion up until some limit where motion occurs. It is that threshold of motion which is characterized by thecoefficientof static friction. The coefficient of static friction is typically larger than the coefficient ofkinetic friction.In making a distinction between static and kinetic coefficients of friction, we are dealing with an aspect of "real world" common experience with a phenomenon which cannot be simply characterized. The difference between static and kinetic coefficients obtained in simple experiments like wooden blocks sliding on wooden inclines roughly follows the model depicted in thefriction plotfrom which the illustration above is taken. This difference may arise from irregularities, surface contaminants, etc. which defy precise description. When such experiments are carried out with smooth metal blocks which are carefully cleaned, the difference between static and kinetic coefficients tends to disappear. When coefficients of friction are quoted for specific surface combinations are quoted, it is the kinetic coefficient which is generally quoted since it is the more reliable number.Kinetic frictionKinetic (or dynamic) friction occurs when two objects are moving relative to each other and rub together (like a sled on the ground). The coefficient of kinetic friction is typically denoted as k, and is usually less than the coefficient of static friction.Examples of kinetic friction:Sliding friction(also called dry friction) is when two objects are rubbing against each other. Putting a book flat on a desk and moving it around is an example of sliding friction.Fluid frictionis the interaction between a solid object and afluid(liquid or gas), as the object moves through the fluid. Thedragof air on an airplane or of water on a swimmer are two examples of fluid friction. This kind of friction is not only due to rubbing, which generates a force tangent to the surface of the object (such as sliding friction). It is also due to forces that areorthogonalto the surface of the object. These orthogonal forces significantly (and mainly, if relative velocity is high enough) contribute to fluid friction. Fluid friction is the classic name of this force. This name is no longer used in modernfluid dynamics. Since rubbing is not its only cause, in modern fluid dynamics the same force is typically referred to as drag or fluid resistance, while the force component due to rubbing is calledskin friction. Notice that a fluid can in some cases exert, together with drag, a force orthogonal to the direction of the relative motion of the object (lift). The net force exerted by a fluid (drag + lift) is calledfluidodynamic force(aerodynamic if the fluid is a gas, or idrodynamic is the fluid is a liquid).

When two surfaces are moving with respect to one another, the frictional resistance is almost constant over a wide range of low speeds, and in thestandard modelof friction the frictional force is described by the relationship below. Thecoefficientis typically less than the coefficient ofstatic friction, reflecting the common experience that it is easier to keep something in motion across a horizontal surface than to start it in motion from rest.

Friction PlotStatic frictionresistance will match the applied force up until the threshold of motion. Then thekinetic frictional resistancestays about constant. This plot illustrates thestandard modelof friction.

The above plot, though representing a simplistic view of friction, agrees fairly well with the results of simple experiments with wooden blocks on wooden inclines. The experimental procedure described below equates the vector component of the weight down the incline to the coefficient of friction times the normal force produced by the weight on the incline.

Having taken a large number of students through this experiment, I can report that the coefficient of static friction obtained is almost always greater than the coefficient of kinetic friction. Typical results for the woods I have used are 0.4 for the static coefficient and 0.3 for the kinetic coefficient.When carefully standardized surfaces are used to measure the friction coefficients, the difference between static and kinetic coefficients tends to disappear, indicating that the difference may have to do with irregular surfaces, impurities, or other factors which can be frustratingly non-reproducible. To quote a view counter to the above model of friction:"Many people believe that the friction to be overcome to get something started (static friction) exceeds the force required to keep it sliding (sliding friction), but with dry metals it is very hard to show any difference. The opinion probably arises from experiences where small bits of oil or lubricant are present, or where blocks, for example, are supported by springs or other flexible supports so that they appear to bind." R. P. Feynman, R. P. Leighton, and M. Sands, The Feynman Lectures on Physics, Vol. I, p. 12-5, Addison-Wesley, 1964.Other types of friction

Rolling resistanceMain article:Rolling resistanceRolling resistance is the force that resists the rolling of a wheel or other circular objects along a surface. Generally the force of rolling resistance is less than that associated with kinetic friction.[5]Typical values for the coefficient of rolling resistance are 0.001.[6]One of the most common examples of rolling resistance is the movement ofmotor vehicletireson aroad, a process which generatesheatandsoundas by-products.[7]

Triboelectric effectRubbing dissimilar materials against one another can cause a build-up ofelectrostatic charge, which can be hazardous if flammable gases or vapours are present. When the static build-up discharges,explosionscan be caused by ignition of the flammable mixture.

Reducing friction

DevicesDevices such as tires,ball bearings, air cushion or roller bearing can change sliding friction into a much smaller type of rolling friction. Manythermoplasticmaterials such asnylon,HDPEandPTFEare commonly used for low friction bearings. They are especially useful because the coefficient of friction falls with increasing imposed load.

LubricantsA common way to reduce friction is by using alubricant, such as oil, water, or grease, which is placed between the two surfaces, often dramatically lessening the coefficient of friction. The science of friction and lubrication is calledtribology. Lubricant technology is when lubricants are mixed with the application of science, especially to industrial or commercial objectives.Superlubricity, a recently-discovered effect, has been observed ingraphite: it is the substantial decrease of friction between two sliding objects, approaching zero levels. A very small amount of frictional energy would still be dissipated.Lubricants to overcome friction need not always be thin, turbulent fluids or powdery solids such as graphite andtalc;acoustic lubricationactually uses sound as a lubricant.

Energy of frictionAccording to the law ofconservation of energy, no energy is destroyed due to friction, though it may be lost to the system of concern. Energy is transformed from other forms into heat. A sliding hockey puck comes to rest because friction converts itskinetic energyinto heat. Since heat quickly dissipates, many early philosophers, includingAristotle, wrongly concluded that moving objects lose energy without a driving force.When an object is pushed along a surface, the energy converted to heat is given by:whereFn is the normal force,k is the coefficient of kinetic friction,x is the coordinate along which the object transverses.Physicalwearis associated with friction. While this can be beneficial, as inpolishing, it is often a problem. As materials are worn away,fitand finish of a object can degrade until it no longer functions properly.[8]

Work of frictionIn the reference frame of the interface between two surfaces, static friction always does nowork, because there is never any displacement. In the same reference frame, kinetic friction is always in the direction opposite the motion and so does negative work.[9]However, friction can do positive work in certaininertial frames of reference. One can see this by placing a heavy box on a rug, then pulling on the rug quickly. In this case, the box slides backwards relative to the rug, but moves forward relative to the floor, an inertial frame of reference. Thus, the kinetic friction between the box and rug accelerates the box in the same direction that the box moves, doing positive work.[10]The work done by friction can translate into deformation, wear, and heat that can affect the contact surface's material properties (and even the coefficient of friction itself). The work done by friction can also be used to mix materials such as in the technique offriction welding.

Question :During dry weather, rubbing a plastic scale with dry hair, attracts small pieces of paper. This is due to

Answer :

gravitational force

electric force

frictional force

muscular force

Question :Which of the following is an example of contact force ?

Answer :

Magnetic force

Muscular force

Electric force

Gravitational force

Question :A batsman hits a cricket ball which then rolls on the level ground. After covering a short distance the ball comes to rest. The ball stops due to

Answer :

magnetic force

frictional force

gravitational force

muscular force

Question :In liquids, the pressure

Answer :

increases with depth

decreases with depth

remains same at all depths

sometimes increases sometimes decreases

Question :When two unbalanced forces act on a body, in opposite directions, the net force is equal to

Answer :

the sum of the individual unbalanced forces.

zero

difference between the two unbalanced forces and is in the direction of the larger force.

difference between the two unbalanced forces and is in the direction of smaller force

MCQs Based On Thrust , Presure,Buyoancy and Density

1. Units of pressure are :-

(a) N/m2 (b) m2/N

(c) Nm2 (d) Nm

2. Units ofRelative Densityare :-

(a) Kg/m3 (b) Unit less

(c) Depends on the density of the substance

(d) Depend on the density of water

3. Pressure at a point in the liquid is

(a) Same in all directions (b) Greater in the upward direction

(c) Grater in the downward direction (d) None of the Above

4. If the area of an object is less then the pressure acting on that object will be

(a) Less (b) More

(c) Independent of area (d) None of the above

5. If cross sectional area of an object is more than the pressure applied by the external force is :-

(a) Less (b) More

(c) Remains same (d) None of the above.

6. Mass per unit volume of a substance is called

(a)Density (b)Relative density

(c)specific gravity (d)None

7. Abuoyant forceof 200 g wt. acts on a body dipped completely in water. If the apparent weight of body is 100 g wt., then its actual weight is

(a)200 g wt. (b)100 g wt.

(c)300 g wt. (d)None of these

8. If the weight of the floating body is equal to the buoyant force then body

(a)Sinks (b)Rises

(c)Floats (d) First floats and then sinks

9. When a body is placed in a liquid, the buoyant force experienced by it is equal to the

(a)weight of liquid displaced (b)Density of the body

(c)volume of body (d)Density of liquid

10. A body immersed in a liquid will rise to the surface if the buoyant force acting on it is

(a)greater than its actual weight (b)less than its actual weight

(c)equal to its actual weight (d)equal to Zero

11. When a body is submerged in a liquid, its weight

(a)Remains same (b)Increases

(c)Decreases (d)Reduces to zero

12. A body weighing 200 g wt. is dipped in water. Its weight in water as indicated by thespring balanceis 150 g wt. The buoyant force acting on the body is

(a)200 g wt (b)1000 g wt

(c)50 g wt (d)250 g wt

13. A body is floating in upright position in water. Then force of gravity and buoyant force acting on the body are

(a) In the same direction long the same line

(b) In opposite direction but along the same line

(c) At right angle to each other.

(d) In opposite to direction and from couple which turns and only anti-clockwise.

14. A body floats in vertical position, when its centre of buoyancy is

(a) Below C.G. of the body (b) Above the C.G. of the body

(c) At some place where the C.G. of the body lies (d) None of these.

15. In case of a body placed in liquid if the buoyant force is equal to its acutal weight, then its apparent weight

(a) Is equal to the actual weight (b) Is equal to buoyant force

(c) Is greater than its actual weight (d) Is equal to zero

16. A block of ice is floating on water contained in a beaker. When all theice melts, the level of water

(a)rises (b)falls

(c)remains unchanged(d)None

17. Aboat floatingin a tank is carrying passengers. If the passengers drink water, the water level of the tank

(a) rises (b) falls

(c) remains unchanged (d) depends upon the atmospheric pressure

CBSE Class 8 - Science - CH11 - Force and PressureForce and PressureNCERT Chapter Solutions and Q & AQ1: Give two examples each of situations in which you push or pull to change the state ofmotion of objects?Answer:A goal keeper saving a goal. Hockey player flicking a ball.Q2: What is a force?Answer: Force is a push or a pull acting on an object which changes or tends to change the state of the object. Actions like pulling, pushing, lifting, kicking, opening etc. are tasks we generally do in our daily life. These actions often result in change in position of the object. We say an effort is required either to push or pull an object, this effort is called force. The SI (International System of Units) unit of force is NewtonQ3: Give two examples of situations in which applied force causes a change in the shape of an object.Answer: a. A lump of dough on a plate. b. A rubber band suspended from a hook.Q4: Fill in the blanks in the following statements:Answer: (a) To draw water from a well we have to pull (force) at the rope. (b) A charged body attracts an uncharged body towards it. (c) To move a loaded trolley we have to push it. (d) The north pole of a magnet repels the north pole of another magnet.Q5: An archer stretches her bow while taking aim at the target. She then releases the arrow, which begins to move towards the target. Based on this information fill up the gaps in the following statements using the following terms: muscular, contact, non-contact, gravity, friction, shape, attractionAnswer:(a) To stretch the bow, the archer applies a force that causes a change in its shape.(b) The force applied by the archer to stretch the bow is an example of muscular force.(c) The type of force responsible for a change in the state of motion of the arrow is anexample of a contact force.(d) While the arrow moves towards its target, the forces acting on it are due to gravityand that due to friction of air.Q5: What is the relation between Force and interaction?Answer: Whenever there is an interaction between two objects, there is a force acting 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.Q6: In the following situations identify the agent exerting the force and the object on which it acts. State the effect of the force in each case.Answer:S. No.Situation Agent Object Effect 1 Squeezing a piece of lemon between the fingers to extract its juice Fingers lemon The effect of the force is that the juice is extracted from the lemon 2 Taking out paste from a toothpaste tube. Fingers toothpaste tube The effect of the force is that the toothpaste is coming out from the tube 3 A load suspended from a spring while its other end is on a hook fixed to a wall. load spring The effect of the force is that the spring expands as the other end is on a hook fixed to a wall.4An athlete making a high jump to clear the bar at a certain height. Athlete athletes body The effect of the body is that the athlete will jump on the other side of the bar at a certain height.Q6: A blacksmith hammers a hot piece of iron while making a tool. How does the force due to hammering affect the piece of iron?Answer: The force due to hammering changes the shape of the piece of iron.Q7: What force can do?Answer: When a force is applied on an object, it: can move an object from rest. can change the speed of an object if it is moving. can change the direction of motion of an object. can bring about a change in the shape of an object. may or may not cause any of these or all of these effects.Q8: An inflated balloon was pressed against a wall after it has been rubbed with apiece of synthetic cloth. It was found that the balloon sticks to the wall. Whatforce might be responsible for the attraction between the balloon and the wall?Answer: The force which is responsible for the attraction between the ball and the wall iselectrostatic force. When we rub the balloon by a synthetic cloth, it gets charged. When itis taken near the wall, it will get attracted towards the uncharged wall because of theelectrostatic force which is the force exerted by a charged body on another unchargedbody.Q9: Name the forces acting on a plastic bucket containing water held aboveground level in your hand. Discuss why the forces acting on the bucket do notbring a change in its state of motion.Answer: The force acting on a plastic bucket containing water held above the ground levelare :1. Gravitational force: It is acting downwards.2. Muscular force: It is applied by our hands to lift the bucket in upward direction.Although these forces are acting on the bucket but no change is found in its state ofmotion because the forces are balancing each other and as a result net force becomeszero.Q10: A rocket has been fired upwards to launch a satellite in its orbit. Name thetwo forces acting on the rocket immediately after leaving the launching pad.Answer: Two forces acting on the rocket immediately after leaving the launching pad are:1. Frictional force due to air,2. Force of gravity pulling in downward direction.Q11: When we press the bulb of a dropper with its nozzle kept in water, air in thedropper is seen to escape in the form of bubbles. Once we release the pressureon the bulb, water gets filled in the dropper. The rise of water in the dropper isdue to(a) pressure of water(b) gravity of the earth(c) shape of rubber bulb(d) atmospheric pressureAnswer: Atmospheric pressureQ12: What are balanced forces andunbalanced forces?Answer: Balanced Forces: Two or more forcesexerted on an object are balanced forces if theireffects cancel each other and they do not cause achange in the object's motion. If the forces on anobject are balanced, the net force is zero.Unbalanced Forces: Two or more forces exertedon an object are unbalanced if their effects do not cance each other. In this case, the netforce is not zero which may result in change in state of motion of the body.Q13: Why does hair of the hand raise when the hand is brought near to televisionscreen once the TV is switched off?Answer: The television screen becomes electrically charged and it exerts an electrostaticforce on the hair of your hand. This force is a non-contact force because, there is nocontact between the screen and the hair.Q14: From the figure below, find out where the object will move in each case?Answer: Case (i): The two forces acting are on the same direction. Thus the Net force isthe sum of these two i.e. F = 5N + 7N = 12N. The net force is non-zero, therefore theobject will experience motion in right direction.Case (ii): In this case, the two forces acting are in opposite direction. Therefore the netforce is the difference of the twoi.e. 5N - 5N = 0N. Since the net force is ZERO, the forces are balanced and its state ofmotion will not change. Initially the object is at rest, it will remain in rest.Case (iii): Two forces acting are in opposite direction. Therefore, net force is 15 - 5N =10N. Since the net force is non-zero, the object will experience a state of motion in thedirection of larger force i.e. Net force of 10N will move the object upwards.Q15: Define Pressure.Answer: The force acting per unit surface area is called pressure. The SI unit of pressureis measured in newton/metre2, which is equal to 1 pascal (Pa).Pressure = Force AreaQ16: How do fluids exert pressure?Answer: Fluids exert pressure in all directions. Fluids exert pressure on all bodiesimmersed in them and also on the walls of the container that holds them.Q17: A force of 50N is applied on an area of 2m2. Compute the pressure beingapplied on the area.Answer: Given, Force = 50Nand Area = 2m2.Pressure = Force/Area = 50/2= 25 Pa.Q18: What is atmosphericpressure?Answer: Air exerts pressureon all objects. The pressureexerted by air is calledatmospheric pressure.Q19: What are the factorspressure exerted by liquidsdepend on?Answer: Pressure exerted byliquids depend upon:The pressure exerted by liquids at the bottom depends on the height (h) of theliquid column.Liquid exerts pressure on the walls of the container.Liquid exerts equal pressure at the same depth.The pressure of the liquid increases with the increase in depth(h).The pressure depends on density(d) of a liquid.The pressure of a liquid depends on gravitational force (g).Mathematically, pressure exerted by liquid can be expressed as:p = h d gwhere,p = pressure of a liquid,h = height of the liquid column.d = density of liquid andg = gravitational force.Q20: Name the instrument used to measure atmospheric pressure.Answer: Barometer.Q21: Why do sea divers wear specially designed suits?Answer: Since the pressure of liquid increases with an increase in depth. Speciallydesigned suits protect scuba divers from the huge pressure of the water underneath.Q22: Who invented the first barometer?Answer: Torricelli, an Italian Scientist (1643)Q23: There is no atmosphere around many planets, then why there is atmospherearound earth?Answer: Earth and some inner planets have atmosphere due to force of gravity on theplanet. Due to force of gravity, gases in atmosphere do not escape. In case of the moonand some other planets, the force due to gravity is comparatively too weak to hold thegases.Q24 Have you been to a place where you feel atmospheric pressure?Answer: When we go higher at mountains or in an aeroplane during flight, our ears pop.As we go higher, the atmospheric pressure decreases, and to balance internal bodypressure our ears pop. Stronger winds also due to low pressure region created.(MCQs)Q1: Opening a door is an example of ._______(a) Contact force(b) Non contact force(c) Spring force(d) Magnetic forceQ2: What is the cause of change in motion or change in the state of motion?(a) Pressure(b) Atmospheric Pressure(c) Friction(d) ForceQ3: A charged balloon attaches to a wall because of ______(a) Muscular force(b) Electrostatic force(c) Magnetic force(d) Gravitational forceQ4: The unit of force is _______(a) Newton(b) Dyne(c) kg. weight(d) All of theseQ5: What is force?(a) Push(b) Pull(c) both a and b(d) None of theseQ6: A body is said to be under balanced forces when the resultant force appliedon that body is_____(a) One(b) Zero(c) Infinite(d) None of theseQ7: The ratio of force, acting perpendicular to the area, on which it acts is knownas ________.(a) Force(b) Friction(c) Presuure(d) DensityQ8: The pressure of the water at the bottom of the pond is _______ at the surfaceof the pond.(a) Lower than(b) Higher than(c) Same(d) either lower or higherQ9: As we go to the higher altitude the atmospheric pressure ________.(a) Increases(b) Decreases(c) Remains same(d) Cannot sayQ10: The force involved in falling an apple from a tree is:(a) Magnetic force(b) Contact force(c) Electrostatic force(d) Gravitational forceAnswers:1: (a) Contact force2: (d) Force3: (b) Electrostatic force4: (d) All of these5: (c) both a and b6: (b) Zero7: (c) Presuure8: (b) Higher than9: (b) Decreases10: (d) Gravitational force