is friction a retarding or accelerating force.in ncert class 11 part 1 it is written that it is an accelerating force but how is this true.how can friction accelerate a body.

Asked by architsrivastava02 | 3rd Oct, 2010, 12:55: PM

Expert Answer:

Friction is always retarding force.Though it can provide acceleration to some other point ,at which it is not acting directly.
To understand this,consider the above fig.
A disc (Centre of mass at O)  rotating clockwise, is put on the surface S at point B.At any particular instance the poin A of disc comes in contact with point B on surface.The relative motion between A & B is :-  A is moving left relative to B.
Therefore to oppose this relative motion friction f act on A towards right (see fig).
So f is trying to slowdown A.But when we consider disc as a system ,a net force f is acting on it ,so the disc will start moving (accelerating) towards right .Notice that motion of point A relative to B is towards left,But that of O relative to B is towards right.
Hence main work of f is to slowdownd relative motion between points of contact (here A & B ) and due the introduction of f in the system some other point of body (here O) starts (or may start) accelerating.
So
Frictional forces always act in such a way so that relative motion of points of contact could be decreased. Hence its always retarding force for the point, where it is acting (here A)

Answered by  | 26th Oct, 2010, 09:39: PM

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