Question answers -3 of ch-3(force) of class-6

 21. When you apply the brakes the bicycle stops and the rim of the wheel becomes hot. explain the reason.

Ans.The rim of the bicycle becomes hot because of the frictional force between the brake shoe and the rim.When we applys brake then brake shoe gets pressed against the rim, and starts opposing motion of the wheel.we know that friction produces heat. Thus the rim of the wheel becomes hot.

22. The eraser get smaller and smaller as you use it more and more. explain the reason.

Ans.The eraser get smaller and smaller as you use it more and more. the reason is that the frictional force between eraser and the the surface.we know that friction causes wear and tear.so size of eraser decreases.

23. List three ways of of reducing friction.

Ans. The three ways of reducing friction are as follows:

1. By making the surfaces smooth by polishing them.

2. By use of lubricants such as oil and grease friction in machines are reduced. Oil or grease is applied between the moving parts of the machine this fills the fine pores or depressions in the surfaces and thus reduces friction.

3. By the  use of ball bearings in place of wheels and axles.since rolling friction  is much less than the sliding friction.

24.It is difficult to open an ink pot with greasy or oily hands .Explain.

Ans. It is difficult to open and ink pot with greasy or oily hands because in order to open an inkpot we have to grip it properly but since there is grease or oil is present on its surface so there the friction is very less and in absence of the minimum frictional force,we are unable to hold it properly and hence our hand  gets slip and we find it difficult  to open inkpot.

25. It is difficult to walk on a wet Road .Explain.

Ans. It is is difficult to walk on a wet Road because on a wet Road there frictional force becomes very less and in absence of the minimum frictional force, a person can not walk properly.we know that to walk on a surface there must be a minimum frictional force between the foot and the surface.

26. Give three examples to illustrate that friction is a necessary evil.

Ans. Friction is is necessary  to do our daily activities but at the same time it is undesirable if it exceeds its limit.

1. Brakes in a vehicle work because of the friction between the brake -lining and the drum of the wheel.

2. Friction is necessary to burn a matchstick.

3. We walk on on road because of the minimum frictional force between the foot and the surface.

And at the same time friction causes wear and tear in the moving parts , produces heat & decreases efficiency.

So we may conclude that friction is must for us to do our basic activities of life.we can not live without it.Thus friction is a necessary evil.

27.Define :

a)static friction b)sliding friction c)rolling friction

Ans. a) static friction-The maximum friction force exerted by a surface on a body so long as it remains stationary is called the satatic friction.

b)sliding friction-

The force exerted by the surface on the object while it is sliding is called the sliding friction.

c)Rolling friction-

The force which opposes the rolling motion of the object is called the rolling friction.

28.Arrange the following in descending order:a)static friction b)sliding friction c)rolling friction

Ans. Static friction> sliding friction>Rolling friction.

29.A body needs a force F1 just to start motion on a surface, a force F2 to continue its motion and a force F3 to roll on the surface.what is a)xtatic friction b)sliding friction c) rolling friction?state whether F2cis eqyal, less than or greater than F1 & F3.

Ans.F1  - static friction

F2  -sliding friction

F3  -rolling friction

F2<F1

F2>F3.