Q.1.    A book, kept on a tilted desk, starts sliding down. What will be the direction of frictional force acting on it?

Ans. When a book kept on tilted writing desk starts sliding down, then the direction of frictional force will be in the opposite direction. Thus, frictional force will act in upward direction.

Q.2.    Sheila spills a bucket of soapy water on a marble floor accidentally. Would it make it easier or more difficult for her to walk on the floor? Why?

Ans. After spilling a bucket of soapy water on a marble floor, it would be more difficult for Sheila to walk on the floor. The layer of soapy water reduces the friction and the foot cannot make a proper grip on the floor.

Q.3.    Explain why sportsmen use shoes with spikes.

Ans. Sportsmen use shoes with spikes in order to avoid getting slipped in the field. The spikes in the shoes increase the friction, so sportsmen can run on the ground easily.

Q.4.    Rajendra has to push a lighter box and Shankar a has to push a similar heavier box on the same floor. Who will have to apply a larger force and why?

Ans. The heavy object will be pressed hard against the opposite surface and produces more friction. So, Shankar will have to apply a larger force due to more friction.

Q.5.    Explain why sliding friction is less than the static friction.

Ans. We know that friction is caused by the interlocking of irregularities in the two surfaces. In case of static friction, interlocking between two objects is very strong but in case of sliding friction, there is a less time of getting interlocked with each other. Thus, the sliding friction is less than the static friction.

Q.6.    Give examples to show that friction is both a friend and a foe.

Ans. Friction as a friend

(i) It allows us to walk comfortably.

(ii) It allows us to minimise the speed or to stop the moving objects.

(iii) It allows us to write on a board with chalk.

Friction as a foe

(i) It causes wear and tear in objects.

(ii) It causes damage to the parts of machines.

(iii) It does not allow the free movement of objects.

Q.7.    Explain why objects moving in fluids must have special shapes.

Ans. When objects move through fluids, they have to overcome frictional force (drag) acting on them. In this process, they lose their energy. Efforts are made to minimise friction. So, objects are given special shapes and this type of shape is known as streamlined shape.

Q.8.    Figure shows two boys applying force on a box. If the magnitude of the force applied by each is equal, will the box experience any force of friction?

Ans. No, the force applied by both boys is equal. So, net force will be zero and hence friction force will not come into play.

Q.9.    It is easier to drag a carpet from the floor, but it is difficult to drag the carpet when someone is sitting on it. Explain the reason behind it.

Ans. If the two surfaces are pressed harder, then the force of friction will increase. So, it is easy to drag a carpet when nobody is sitting on it but when a person is sitting on it, then it produces more friction.

Q.10.         Give a reason for calling friction as a necessary evil.

Ans. Friction is a necessary evil because it causes a lot of wear and tear.

Q.11.         Mention the moment when the rolling friction comes into play.

Ans. The rolling friction comes into play when one body rolls over the surface of another.

Q.12.         The pieces of luggage fitted with the rollers. Explain why.

Ans. In order to reduce single friction, pieces of luggage are fitted with rollers so that even a child can pull the luggage easily.

Q.13.         Can we reduce friction to zero by polishing surfaces or using large amount of lubricants?

Ans. No, friction can never be entirely eliminated. Some irregularities are always there.

Q.14.         Is frictional force a contact force?

Ans. Yes, because friction acts only when two surfaces are in contact with each other.

Q.15.         Why it is difficult to move on a wet marble floor?

Ans. On a wet marble, floor, frictional force between the foot and floor decreases making it slippery, thus we feel difficult to move on a wet marble floor.

Q.16.         Why does a matchstick catch fire when it rubbed on rough surface?

Ans. On rubbing the matchstick on a rough surface, the temperature increases due to friction, making the matchstick catch fire.

Q.17.         What do we call the substances that are used to reduce friction?

Ans. The substances that are used to reduce friction between two surfaces are called lubricants.

Q.18.         Name the force which helps to write by chalk on blackboard.

Ans. Frictional force helps to write by chalk on blackboard.

Q.19.         It is difficult to move on a wet marble floor. Why?

Ans. It is difficult to move on a wet marble floor because the frictional force between our foot and wet marble floor is very low.

Q.20.         List the parameters on which force of friction depends.

Ans. Frictional force depends upon smoothness/roughness of surfaces and their mass.

Q.21.         Name the type of surfaces that have more irregularities.

Ans. Rough surfaces have more irregularities.

Q.22.         A jar of a mixture becomes hot when mixer is run for few minutes. Comment.

Ans. Due to friction between contents and the wall of the jar, the jar of the mixture becomes hot when miser is run for few minutes.

Q.23.         Will force of friction come into play, when a raindrop rolls down a glass window pane?

Ans. Yes, friction comes into play when two surfaces are in contact, e.g. glass and water in this case.

Q.24.         Explain fluid friction in brief.

Ans. When an object moves in a fluid, then the resistance to the movement of object, exerted by fluid is called fluid friction.

Q.25.         Which is the special shape of the objects moving in fluids?

Ans. Streamlined shape is the special shape given to objects moving in fluids.

Q.26.         Liquids and gases are called as fluid. Why?

Ans. Liquids and gases can flow and they do not have definite shape. So, they are called as fluid.

Q.27.         Fishes move easily in water. How?

Ans. Fishes are having streamlined shape, so they can move easily in the water.

Q.28.         Why are oil, creams and grease called lubricants?

Ans. Oil, creams and grease when applied on a surface make it smooth, thus reduce friction, so these are called lubricants.

Q.29.         Can we eliminate friction completely?

Ans. No, we cannot eliminate friction completely because it is the characteristic property (molecular property) of material.

Q.30.         Why do kabbadi players rub their hands with soil?

Ans. Kabbadi players rub their hands with soil to increase the friction so that they can grip the opposite team players firmly.

Q.31.         If a body is moving on rough horizontal surface towards east, then in which direction the frictional force will act?

Ans. Frictional force always act in the direction opposite to the direction of motion, so it will act towards west.

Q.32.         Why an arrow with a pointed tip can move faster in air than that with a flat tip?

Ans. The arrow with a pointed tip suffers small air resistance due to its pointed shape, so it moves faster.

Q.33.         If there was no friction, then what would happen when an object starts moving?

Ans. The object would never stop if there was no friction.

Q.34.         Is the friction same for all the surfaces?

Ans. No, because friction depends on the nature of surface in contact.

Q.35.         State the reason, why do we slip when we step on a banana peel.

Ans. As we step on a banana peel, then there is no grip between our foot and the banana peel due to very less friction. So, we slip accidentally.

Q.36.         You might have noticed that when used for a long time, slippers with rubber soles become slippery. Explain the reason.

Ans. It is due to continuous rubbing of soles with the ground, the spikes on the sole get damaged slowly and the soles become slippery.

Q.37.         Name any of the five smooth surfaces from your surroundings.

Ans. The five smooth surfaces are

(i) Glazed tiles

(ii) Surface of an oily plot

(iii) Surfaces of wet soaps

(iv) Mirror surface

(v) Floor of polished room

Q.38.         Explain the reason of rubbing the hands in winters.

Ans. As rubbing of hands against each other causes the friction between them and it produces heat which makes us warm. That's why we rub our hands to make them warm when we feel cold.

Q.39.         Cartilage is present in the joints of our body which helps in their smooth movement. With advancing age, this cartilage wears off. How would this affect the movement of joints?

Ans. Cartilage present in the joints of our body, reduces friction during movement of joints. But on wearing off this cartilage, the force of friction increases due to which the smoothness of movement decreases and one feels the joint pain.

Q.40.         Is there a force of friction between the wheels of a moving train and iron rails? If yes, name the type of friction. If an air cushion can be introduced between the wheel and the rail, what effect will it have on the friction?

Ans. Yes, there is always a force of friction between the wheels of a moving train and iron rails. The name of this friction is rolling friction, since, the wheels are rolling on the track. On introducing air cushion, the frictional force becomes less, since there is no contact between rails and wheels.

Q.41.         While playing tug of war, Rashi felt that the rope was slipping through her hands. Suggest a way out for her to prevent this.

Ans. To prevent slipping of the rope from hands, Rashi has to make her hands somewhat non-smooth, so she can rub her hands by introducing the sand, between them.

Q.42.         Explain in brief, how does the friction get affected by the nature of surface.

Ans. Since, in order to move an object on any surface, we have a force to overcome the interlocking of surfaces. On smooth surface, there is less number of irregularities while on rough surface large numbers of irregularities, are found. So, as the roughness increases, the force of friction increases.

Q.43.         Is it possible to walk on a frictionless surface? Explain.

Ans. No, we cannot walk on a frictionless surface. It is only the friction which helps us to keep our feet stuck and walk on the surfaces. The grooves of our feet and shoes are locked into the irregularities of the floor or surface and enable us to walk.

Q.44.         The handle of a cricket bat or a badminton racquet is usually rough. Explain the reason.

Ans. The handle of a cricket bat or badminton racquet is rough, so that while playing, the bat or badminton racquet does not slip away from the hands of the player. Roughness is responsible for the frictional force without which fine gripping will not be possible.

Q.45.         There is more drag on an object when it moves through water than when it moves through air. Explain with the reason.

Ans. Fluid friction depends on the viscosity (or thickness) of fluid. Higher the viscosity of fluid, greater will be the frictional force acting on an object moving through it. As we know that water is much more viscous than air, so there will be much more frictional force on an object when it moves through water than when it moves through air.

Q.46.         Explain why the surface of mortar and pestle (silbatta) used for grinding is etched again after prolonged use.

Ans. After prolonged use, the mortar and pestle lose the roughness, due to which frictional force reduces and it does not work properly. So, we have to etch it to make it rough again.

Q.47.         A marble is allowed to roll down an inclined plane from a fixed height. At the foot of the inclined plane, it moves on a horizontal surface (a) covered with silk cloth (b) covered with a layer of sand and (c) covered with a glass sheet. On which surface, will the marble move the shortest distance. Give reason for your answer.

Ans. Marble will move the shortest distance on the layer of sand because it will exert a greater force of friction on the marble and other two surfaces like silk cloth and glass sheet will exert a lesser friction force comparatively.

Q.48.         A father and son pushed their car to bring it to the side of road as it had stalled in the middle of the road. They experienced that although they had to push with all their might initially to move the car, the push required to keep the car rolling was smaller, once the car started rolling. Explain.

Ans. When the car is at rest, we have to apply greater force because the value of static friction is more. As the car starts moving, the friction changes into the rolling friction which is always less than the previous one. So, we have to exert the lesser force to keep it in motion.

Q.49.         Explain why

(a) a pencil will write on paper but not on glass.

(b) climbing a greasy pole is very difficult.

Ans.

(a) Due to friction, a pencil writes on paper but there is smoothness on the surface of glass, a pencil cannot write on glass.

(b) If a pole is polished with grease, then the surface of pole becomes smoother thus reducing friction. So, climbing a greasy pole becomes tough.

Q.50.         Explain which principle is involved in machine belts.

Ans. Since, in many machines (like flour mills), belts are used to drive wheels for running the machines. Greater friction is required between the belts and machine wheels so that the belts can drive the machine wheels properly without slipping off wheels. So, in order to increase friction, machine belts are made up of special materials having rough surfaces.

Q.51.         It is easier to move the box from rest or to move it when it is already in motion. Explain why.

Ans. When the object is at rest, we have to apply more force to interlock the irregularities. On the other hand, when the object starts moving the contact points on its surface do not get enough time to lock into the contact points on the ground. So, it is easier to move the block when it is already in motion.

Q.52.         Give two examples to reduce friction.

Ans. The two examples to reduce friction are

(i) Drops of oil are poured on the hinges of a door to move the door smoothly.

(ii) Fine powder are sprinkled on the carrom board to reduce friction.

Q.53.         Two boys are riding their bicycles on the same concrete road. One has new tyres on his bicycle while the other has tyres that are old and used. Which of them is more likely to slip while moving through a patch of the road which has lubricating oil spilled over it?

Ans. The boy having the tyres which are old and used is most likely to slip because these tyres will experience less frictional force which is insufficient to move on the oily road.

Q.54.         Imagine that an object is falling through a long straight glass tube held vertical, air has been removed completely from the tube. The object does not touch the walls of the tube. Will the object experience any force of friction?

Ans. No, the object will not experience any frictional force because to experience the force of friction, two surfaces must be there and there is only one surface in this case.

Q.55.         Two friends are trying to push a heavy load as shown in figure below. Suggest a way which will make this task easier for them.

Ans. Both the friends can put· rollers below the heavy load. Since, the rolling friction is smaller than the sliding friction. Therefore, putting rollers below the heavy load will make this task easier for them.

Q.56.         When the cutting edge of a knife is put against a fast rotating stone to sharpen it, sparks are seen to fly. Explain the reason.

Ans. Due to the friction between knife and stone, the temperature of the knife and hence stone increases and it increases to such a level that the sparks are produced which can be seen while sharpening it.

Q.57.         While travelling on a rickshaw, you might have experienced that if the seat cover is very smooth, you tend to slip when brakes are applied suddenly.

Ans. If the seat cover of rickshaw is very smooth, then 'the friction between our body and the seat is very small. Therefore, when the brakes are applied, we tend to slip.

Q.58.         Explain with the help of an example that heat is produced by friction.

Ans. As friction opposes the relative motion between the two surfaces also produce heat. So, when we rub our hands together vigorously for few minutes, we feel hotness when we strike a matchstick against the rough surface due to which it catches fire.

In machineries also, heat is produced due to friction as the machine parts rub against each other.

Q.59.         We have two identical metal sheets. One of them is rubbed with sand paper and the other with ordinary paper. The one rubbed with sand paper shines more than the other. Give reason.

Ans. While rubbing with sand paper, more frictional force is produced between the layers of metal sheet and sand paper which causes more force on dust particles and they are removed easily, so it will shine more. But in case of ordinary paper, the force of friction is not sufficient to remove all the dust, so it will shine less in this case.

Q.60.         Explain some examples where we have to reduce the friction.

Ans. When a fine powder sprinkled on the carrom board and a few drops of oil are poured on the hinges of a door, the door moves smoothly. A cycle or a motor mechanic sometimes uses grease between the moving parts of the cycle. In all the above cases, our try was to reduce friction in order to increase efficiency.

Also, interlocking of irregularities is avoided to a great extent due to which the movement becomes smooth. The substances which reduce friction are called lubricants. In some machines, it may not be advisable to use oil as lubricant. There is an air cushion between the moving parts, that is used to reduce friction.

Q.61.         In physics classroom a teacher told to his students that when a cricket player wants to catch a ball then first, he rubs the hands. Thus, there is less chance of slipping the ball from hands.

(a) Explain the reason of not slipping of ball from hand

(b) If the player makes his hands oily instead of rubbing them. Would he be able to catch the ball?

(c) Mention the type of teaching skills of teacher is being represented here?

Ans.

(a) Since, the player rubs his hand due to which the palms become warm which absorb the moisture from the surface and make the surface rough. So, due to roughness of hands, the ball may get a better grip between the hands and does not slip.

(b) Since, the player makes the palm surface oily. So, the number of irregularities decreases and the surface becomes smooth. So, the ball slips off easily from the hand.

(c) As, teacher wants to teach the students through easy example. So, this act of teacher shows the experience and ideality of teacher.

Q.62.         Few days back, Shradha, a student of class VIII was driving the car. She went to a crop field. The level of road was just above the level of ploughed field. As she wanted to stop the slow-moving car, the brakes failed to work at that time. Suddenly, she turned the car towards the ploughed field. After sometime, the car came to rest.

(a) Shradha turned the car towards the ploughed field. Explain why.

(b) Explain, how the car came to rest after covering some distance in the ploughed field.

(c) Mention the values that Shradha exhibits here.

Ans.

(a) She turned her car towards the ploughed field to protect herself from an accident.

(b) Since, the car moved from road to the ploughed field due to which the force of friction between the tyre and the ground surface increases suddenly. Because the large amount of irregularities reduces the movement of tyre. Thus, the car came. to a rest.

(c) The values displayed by Shradha are intelligence and awareness.

Q.63.         Show with the help of an experiment that smoothness of surface reduces friction.

Ans. Firstly, hold a pencil or pen. Now, put it on a table. Slightly push the pencil and see how much distance it covers. Put the marking of the point where the pencil stops after covering the distance. Now spread polythene on the same table. Now, again hold the pencil and put it at the same initial point. Now, push the pencil slightly and let it stop on its own as done earlier.

Now, mark the point where the pencil stops. Notice the difference in the distance. In second chance, the pencil has covered more distance. This means it had to face less friction in second attempt. It shows that smoothness of the surface reduces friction.

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