NCERT Solutions for Class 8 Science Chapter 12 : Friction

### 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.

### 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.

### 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.

### 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.

### 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.

### 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.

### 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.

### 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.

### 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.

### 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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