NCERT Exemplar Solution Class 8 Science Chapter 12
NCERT Exemplar Solution Class 8 Science Chapter 12 Friction all Questions Answer Solution. NCERT Exemplar Class 8 Science Chapter 12 Solution PDF.
NCERT Exemplar Solution Class 8 Science Chapter 12: Overview
NCERT Exemplar Solution Class 8 Science Chapter 12 |
|
Board |
NCERT |
Topic |
Exemplar Problem Solution |
Class |
8 |
Subject |
Science |
Chapter |
12 |
Chapter Name |
Friction |
NCERT Exemplar Solution Class 8 Science Chapter 12 Friction
Multiple Choice Questions
1) Whenever the surfaces in contact tend to move or move with respect to each other, the force of friction comes into play
a) Only if the objects are solid.
b) Only if one of the two objects is liquid.
c) Only if one of the two objects is gaseous.
d) Irrespective of whether the objects are solid, liquid or gaseous.
Answer:- d) Irrespective of whether the objects are solid, liquid or gaseous.
The force of friction come into play whether the objects are solid, liquid or gas. It can occur between any of the two objects among them.
2) In fig. 12.1, a boy is shown pushing the box from right to left. The force of friction will act on the box.
a) From right to left (←)
b) From left to right (→)
c) Vertically downwards (↓)
d) Vertically upwards (↑)
Fig. 12.1
Answer:- b) From left to right (→)
The force of friction will act from left to right because friction opposes the motion between surface And the box.
3) To Sharpen the blade of a knife by rubbing it against a surface, which of the following will be most suitable ?
a) Stone
b) Plastic block
c) Wooden block
d) Glass block
Answer:- a) Stone
Rubbing Stone and knife will create a large amount of friction and it will sharpen the blade of knife. Other objects cannot create that amount of Friction with the knife and thus it will not sharpen the blade effectively.
4) A toy car released with the same initial speed will travel farthest on
a) Muddy surface
b) Polished marble surface
c) Cemented surface
d) Brick surface
Answer:- c) Cemented surface.
Cemented surface will allow toy to move further with the same initial speed because it creates the correct amount of force of friction between toy and surface. Other surfaces will increase or lower down the speed with high or low friction between them.
5) If we apply oil on door hinges, the friction will
a) Increase
b) Decrease
c) Disappear altogether
d) Will remain unchanged
Answer:- b) Decrease
Oil is applied as the lubrication, this decreases the friction between the objects.
6) Which of the following statements is incorrect?
a) Friction acts on a ball rolling along the ground.
b) Friction acts on a boat moving on water.
c) Friction acts on a bicycle moving on a smooth road.
d)Friction does not act on a ball moving through air.
Answer:- d) Friction does not act on a ball moving through air.
Friction also acts on the ball when it is moving through the air. Air and ball creates friction.
7) A boy rolls a rubber ball on a wooden surface. The ball travels a short distance before coming to rest. To make the small ball travel longer distance before coming to rest, he may
a)Spread a carpet on the wooden surface.
b)Cover the ball with a piece of cloth.
c)Sprinkle talcum powder on the wooden surface
d)Sprinkle sand on the wooden surface.
Answer:- c) sprinkle talcum powder on the wooden surface.
Ball is coming to rest over a short distance due to the friction, but sprinkling talcum powder over it will reduces the friction and ball will move further easily.
8) In a large commercial complex there are four ways to reach the main road. One of the path has loose soil, the second is laid with polished marble, the third is laid with bricks and the fourth has gravel surface. It is raining heavily and paheli wishes to reach the main road. The path on which she is least likely to slip is
a)Loose soil
b)Polished marble
c)Bricks
d)Gravel
Answer:- d) Gravel
Gravel is the finest sand, this can absorb water effectively within it. Therefore moving from the gravel road will be least likely to slip as compared to other paths.
Very Short Answer Questions
9) Two blocks of iron of different masses are kept on a cemented floor as shown in fig. 12.2 which one of them would require a larger force to move it form the rest Position?
Fig. 12.2
Answer:- As shown in figure 12.2 , 2 unit block is heavier than 1 unit block. heavy block i.e 2 unit block will required larger force to move. And 1 unit block will required less force To move as compared to heavy one.
10) Will force of friction come into play when a rain drop rolls down a glass window pane?
Answer:- yes. Force of friction plays a role when a rain drop rolls down a glass window pane. Water and other fluids exert force of friction on the object. These is also known as drag.
11) 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 skid while moving through a patch of the road which has lubricating oil spills over it?
Answer:- New tyres are treaded or have some grooves on it, these increase the friction and reduce the skidding of tyres. But old worn out tyres cannot form grip over the road and friction decrease there, due to smooth tyres. Therefore worn out tyres can be skid easily.
12) Fig. 12.3 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?
Fig. 12.3
Answer:- No. The box will not experience any force. As the magnitude of the force from both the opposite side is equal, force of friction will become zero. Therefore there will not be any friction.
13) 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?
Answer:- No. The object will not experience any force of friction. the object is not touching the walls of the tubes also the air is removed from the tube, therefore there will not be any force of friction in the tube.
Short Answer Questions
14) You might have noticed that when used for a long time, slippers with rubber soles become slippery. Explain the reason.
Answer:- Soles of shoes and slippers are made with the grooves so that they could provide better grip with the surface, and should not be slippery. But rubber soles when used for longer time, becomes smooth. These smooth slippers cannot form grip and these reduces the friction with the surface. Therefore slippers with rubber soles become slippery.
15) 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?
Answer:- Yes. There is Rolling friction between the moving train and iron rails. These is occurs when one body rolls on another body. An Air introduced between the moving parts is used for the reducing friction. Therefore friction will be decrease.
16) 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?
Answer:- With the increase in the age, the cartilage present in our joints wears off or vanishes. These happens due to the movement of joints for long time and thus friction plays a role their. After wearing off of cartilage, there will be increase in the friction. It gets difficult to move the joints or feel pain in the joints at the time of movement.
17) While playing tug of war (fig. 12.4), preeti felt that the rope was slipping through her hands. Suggest a way out for her to prevent this.
Fig. 12.4
Answer:- The rope was slipping through preeti’s hands due to sweat, while playing. Therefore the friction between the hands and rope decreases. If preeti would rub her hands in the soil then her hands will not be sweaty. And this will increase the friction and grip between the hands and rope.
18) The handle of a cricket bat or a badminton racquet is usually rough. Explain the reason. Answer:- Roughness of the handle of the cricket bat or badminton racquet is for the increasing friction between hands and handles. Also it gives better grip while playing. Due to these roughness, cricketers can hold the bat with grip and without slipping hands over it.
19) Explain why the surface of mortar and pestle (silbatta) used for grinding is etched again after prolonged use?
Answer:- Morter and pestle is mostly made up of heavy stones. These silbatta is used for grinding from the ancient times. These silbatta have an linings or etched the surface so that it could form a friction and should not skid over it. But if used for longer time the surface of both the pestle and mortar become smooth. And these decreases the friction between them. Smooth surface cannot effectively grind the materials or objects. Therefore for increasing the friction these silbatta is etched again. So that it could properly work of grinding.
20) 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.
Answer:- a marble has an smooth surface so it will roll down easily on the smooth surface without friction. Here we have taken a silk cloth, as silk cloth is also smooth, it will form minimum friction with the marble and decrease the speed of it. Over the glass sheet it will move smoothly and easily pass to the larger distance with very less friction. But while moving over the sand, the marble will cover a very small distance, as the sand will form maximum friction with the marble. So it could cover the shortest distance with the sand.
21) A father and a 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.
Answer:- as the car was stalled in the middle of the road, a push with the force is necessary to keep the car rolling. As rolling friction is less than the sliding friction. Therefore as soon as the car started rolling then maximum force will not be needed to push the car and they have to just balance the friction of the car.
Long Answer Questions
22) 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.
Answer:- sometimes a hard friction produces heat. Here between knife and rotating stone the large amount of friction
takes place and it produced heat. As the stone is rotating therefore the Sparks are seen to fly in the air.
23) 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.
Answer:- the sand paper have rough appearance, the ordinary paper is smooth as compared to it. While rubbing the metal sheet over the sand paper, it produces very large amount of friction with the metal. And thus due to large friction between them the dullness over the metal sheets is removed effectively and it starts shining. When we rub the ordinary paper there is minimum friction occurs, as the paper is quite smooth these does not produce large friction, and hence metal sheet appears less shiny as it cannot remove the dullness clearly.
24) While travelling on a rickshaw, you might have experienced that if the seat cover is very smooth, you tend to slip when barkers are applied suddenly. Explain.
Answer:- For maintaining the grip between the body and seat there should be some specific amount of friction, but as the seat cover is very smooth it produces only small amount of friction and thus the body slips when the brakes are applied.
25) Two friends are trying to push a heavy load as shown in fig. 12.5 suggest a way which will make this task easier for them.
Fig. 12.5
Answer:- One method to make the given task easier is to put the rollers below the load. As we put the rollers below the heavy load, it will become very easy to push the heavy load, because rolling friction is very small than the sliding friction. Thus rolling of heavy load will need a very small push and the heavy load will move further.