Explain why sliding friction is less than the static friction.

3

(a)

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

3

(a)

Explain why objects moving in fluids must have special shapes.

3

(a)

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

3

(a)

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?

3

(a)

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

3

(a)

Explain why a pencil will write on paper but not on glass.

3

(a)

Explain which principle is involved in machine belts.

3

(a)

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

3

(a)

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.

3

(a)

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.

3

(a)

What is the cause of friction?

3

(a)

Mention three examples that show that friction produces heat.

3

(a)

Friction is called a necessary evil. Why?

3

(a)

Why is it easy to drag a mat when nobody is sitting on it but difficult when somebody is sitting on it?

3

(a)