NCERT Solutions for Class 8 Science Chapter 12 : Friction

Q:

Explain why sportsmen use shoes with spikes.

A:

A sportsman wears shoes with spikes because these shoes give him a better grip while running. Due to this, force of friction between the shoes and the ground increases.

Q:

Explain why objects moving in fluids must have special shapes.

A:

An object moving through a fluid experiences an opposing force which tries to oppose its motion. This opposing force is also called the drag force. This force depends on the shape of the body. By giving the body a special shape, this force can be minimised. Thus, it becomes easier for the body to move through the fluid.

Q:

Explain why sliding friction is less than static friction.

A:

Friction arises between two objects when irregularities present in the surfaces get interlocked with each other. In the case of sliding, the time given for interlocking is very small. Therefore, interlocking is not strong. Hence, less force is required to overcome from this interlocking. Thus, sliding friction is less than static friction.

Q:

Iqbal has to push a lighter box and Seema has to push a similar heavier box on the same floor. Who will have to apply a larger force and why?

A:

When a heavy object is kept on the floor, the interlocking of irregularities on the surfaces of box and floor become very powerful. It is because the two surfaces in contact are pressed harder. Thus, more force is required to overcome the interlocking. Therefore, to push the heavier box, Seema has to apply a larger force than Iqbal.

Q:

You spill a bucket of soapy water on a marble floor accidently. Would it make it easier or more difficult for you to walk on the floor? Why?

A:

A person can walk because of the friction present between our feet and the ground. In order to walk, the person pushes the ground in the backward direction with his feet and the force of friction pushes it in the forward direction. When there is soapy water spilled on the floor, the force of friction between the ground and feet decreases. Thus, it becomes difficult to walk on the soapy floor.

Q:

Suppose your writing desk is tilted a little. A book kept on it starts sliding down. Show the direction of frictional force acting on it.

A:

When the book slides on the desk, the force of friction acts between the book and the surface of the desk. The direction of frictional force is opposite to the direction of the motion of book and acts in upward direction.

Q:

Four children were asked to arrange forces due to rolling, static and sliding frictions in a decreasing order. Their arrangements are given below. Choose the correct arrangement.

(a) rolling, static, sliding
(b) rolling, sliding, static
(c) static, sliding, rolling
(d) sliding, static, rolling

A:

The correct option is (c).

Explanation: The force of friction acts due to irregularities between two surfaces. Static friction acts when an object is moved from rest. Sliding friction comes into play when an object slides over the surface of another object. In sliding friction, the time given for interlocking is very less. Hence, interlocking is not strong. Therefore, less force is required to overcome this interlocking. Due to this, sliding friction is less than static friction. Also, the area of contact in case of rolling friction is smallest as compared to static or sliding friction. This area of contact changes gradually because of rolling. Thus, rolling friction is lesser than both static and sliding friction. So, the correct sequence is - static, sliding, rolling.

Q:

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

A:

Friction as a Friend:

  1. Due to friction, a person is able to walk.
  2. Friction between the tip of a pen and a paper helps us to write.

Friction as a Foe:

  1. Due to friction, tyres and soles of shoes wear out.
  2. Friction between different parts of machines can generate heat. This can damage the parts of machines.

Q:

Alida runs her toy car on dry marble floor, wet marble floor, newspaper and towel spread on the floor. The force of friction acting on the car on different surfaces in increasing order will be

(a) wet marble floor, dry marble floor, newspaper and towel.
(b) newspaper, towel, dry marble floor, wet marble floor.
(c) towel, newspaper, dry marble floor, wet marble floor.
(d) wet marble floor, dry marble floor, towel, newspaper

A:

The correct option is (a).

Explanation: Frictional force depends on the nature of surfaces in contact. The rougher the surface, the more is the friction between the surfaces in contact. According to roughness present in the given surfaces, we can arrange the force of friction acting between different surfaces as, wet marble floor, dry marble floor, newspaper and towel.

Q:

Fill in the blanks.

(a) Friction opposes the _____________ between the surfaces in contact with each other.
(b) Friction depends on the _____________ of surfaces.
(c) Friction produces __________.
(d) Sprinkling of powder on the carrom board ________ friction.
(e) Sliding friction is ___________ than the static friction.

A:

(a)  Friction opposes the motion between the surfaces in contact with each other.
(b)  Friction depends on the nature of surfaces.
(c)  Friction produces heat.
(d)  Sprinkling of powder on the carrom board reduces friction.
(e)  Sliding friction is less than the static friction.

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