NCERT Solutions for Class 8 Science Chapter 12 Friction are provided here with simple step-by-step explanations. These solutions for Friction are extremely popular among Class 8 students for Science Friction Solutions come handy for quickly completing your homework and preparing for exams. All questions and answers from the NCERT Book of Class 8 Science Chapter 12 are provided here for you for free. You will also love the ad-free experience on Meritnationâ€™s NCERT Solutions. All NCERT Solutions for class Class 8 Science are prepared by experts and are 100% accurate.

#### Question 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.

Friction force opposes the relative motion between the surfaces of any  two objects in contact. The objects may be solid, liquid or gas.
Hence, the correct answer is option (d).

#### Question 2:

In the given figure 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 (←)         (c) vertically downwards (↓)
(b) from left to right (→)         (d) vertically upwards (↑)

Friction force always acts in opposite to the direction of motion of surfaces in contact. By pushing the box, the box will move leftwards, so the friction force will act from left to right direction (→) on the box.
Hence, the correct answer is option (b).

#### Question 3:

To sharpen the blade of a knife by rubbing it against a surface, which of the following will be most suitable?
(a) stone                   (c) wooden block
(b) plastic block       (d) glass block

Stone will provide more friction than others. Thus, it is most suitable among the given options to sharpen a knife.
Hence, the correct answer is option (a).

#### Question 4:

A toy car released with the same initial speed will travel farthest on
(a) muddy surface                    (c) cemented surface
(b) polished marble surface     (d) brick surface

A polished marble surface will provide the least friction to the car relative to others. So, the toy car will move fastest on polished marble surface.
Hence, the correct answer is option (b).

#### Question 5:

If we apply oil on door hinges, the friction will
(a) increase         (c) disappear altogether
(b) decrease        (d) will remain unchanged

Lubricant oil is used in door hinges to decrease the friction.
Hence, the correct answer is option (b).

#### Question 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.

Friction due to air drag acts on the ball moving in air.
Hence the correct answer is option (d).

#### Question 7:

A boy rolls a rubber ball on a wooden surface. The ball travels a short distance before coming to rest. To make the same 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.

Sprinkling talcum powder  on the wooden surface reduces the friction of wooden surface.
Hence, the correct answer is option (c).

#### Question 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.                 (c) bricks.
(b) polished marble.      (d) gravel.

Gravel path will provide sufficient friction to avoid slipping.
Hence, the correct answer is option (d).

#### Question 9:

Two blocks of iron of different masses are kept on a cemented floor as shown in the given figure. Which one of them would require a larger force to move it from the rest position?

Larger force will be required to move heavier block as more friction force will act on heavier block. So, the second block will require larger force to move.

#### Question 10:

Will force of friction come into play when a rain drop rolls down a glass window pane?

Yes, the force of friction comes into play when a raindrop rolls down a glass window pane. The force of friction acts when two surfaces come in contact and they move or try to move relative to each other. Here one surface is a raindrop’s surface and the other is a glass window pane’s surface.

#### Question 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 spilled over it?

Lubricating oil reduces the friction of the road and make it slippery. New tires provides more friction and better grip on the road to avoid skidding of bicycle, while the bicycle with old tires are more likely to skid on the road.

#### Question 12:

The given 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?

As, boys apply equal and opposite forces on the box, the net force on the box is zero, i.e. there will not be any relative motion between the ground and the box, so the box will not experience any friction force.

#### Question 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?

No, the the object will not experience any friction force as it is not in contact of any other surface and air is also absent in the tube. Friction force only acts when the surfaces are in contact.

#### Question 14:

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

Grooved shapes (dot like) in the bottom of the slippers are used to increase the friction and for better grip on the floor.  After their continuous use for long time, the bottom of the slippers gets smoother (roughness decreases) and hence they become slippery i.e. apply less friction.

#### Question 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?

Yes, there is a friction force acting between the rolling object (wheels of the train) and the floor ( iron rail track), termed as "rolling friction". If an air cushion can be introduced between the wheel and the rail, the friction force will decrease.

#### Question 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?

The main function of cartilage is to provide a smooth, lubricated surface for smoother movement of bones at joints. In old age, this cartilage wears off and it becomes rough causing more friction. Due to this, it creates difficulty in movement at joints. Sometimes, it becomes painful for old age people.

#### Question 17:

While playing tug of war in the given figure., Preeti felt that the rope was slipping through her hands. Suggest a way out for her to prevent this.

By applying dry soil on the rope and her hand, Preeti can increase the roughness of the rope and hence, she can increase the friction between hand and the rope.

#### Question 18:

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

A batsman needs better grip of the bat to hit a ball otherwise it will slip. To provide better grip or to increase friction, the handle of a cricket bat or a badminton racquet is made usually rough.

#### Question 19:

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

After prolonged use, the roughness between the surface of mortar and pestle (silbatta) decreases. Therefore, the surface of mortar and pestle (silbatta) used for grinding is etched again after prolonged use to increase the roughness of the mortar and pestle, which makes it more effective for grinding again.

#### Question 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.

A surface with a layer of sand provides more friction and opposes the motion of marble. Hence, marble covers the shortest distance on the surface with sand layer.

#### Question 21:

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.

Rolling friction force is relatively less than the static ( when the object is at rest) friction force. Initially, they had to apply force to set the car in motion but once the car started rolling, they had to apply force only to balance rolling friction of the car, the value of which is very less.

#### Question 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.

When the cutting edge of a knife is put against a fast rotating stone to sharpen it, the surfaces in contact get heated due to friction force. As the friction is very large in this case, a large amount of heat is produced, due to which we see sparks flying.

#### Question 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.

Sand paper provides more friction than the ordinary paper while rubbing against the metal sheets. Thus, the sand paper is able to remove the outer dull layer from the metal sheet more effectively. Thus, the metal sheet rubbed with sand paper shines more than the other.

#### Question 24:

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. Explain.

When the seat cover is very smooth, the friction between our body and the seat is very small. Due to this, while travelling on a rickshaw, when the brakes are applied we tend to slip.

#### Question 25:

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