Work done by friction

vin300

Consider this:A rigid body collides normally with a rigid surface
The momentum of the system at every instant must be conserved, so when the two are in contact in the rest frame of the observer, there must be something with a momentum. The surface or the body still has a momentum, so saying the displacement of the point of contact is zero is an approximation, it actually moves,if the body recoils with no damage at all, it proves that nothing is really rigid.

RohansK

The friction between the ground and the human foot/shoe sole actually holds the foot or keeps it grounded firmly. This is kind of an interlocking between the two surfaces which facilitate the muscles to exert the force to actually push your body forward.

Had the friction been overcome you would keep on slipping on the ground and forward motion wouldnt be possible. ( just as the case of a car tyre slipping on a mucky road)

So the frictional force is at the base point of contact and this point of contact does not move. So the friction does not do any work. It is the muscular energy that does the work of pushing your body forward.

vin300

How is this a reply to my post?

RohansK

This thread is not started by you, and my answer was for the original poster. who asked a question regarding the work done by frictional force while walking.

I did not answer your question.

Dont be in a hurry it to willbe addressed.

vin300

I remember.Consider this hypothetical situation in which the rigid surface is connected only to rigid objects so according to general interpretation there is no scope of it moving.Now an elastic body colldes this surface so the conservation of momentum requires when its further part is still, the back part must have its whole momentum(thus greater velocity) and it compresses.When the back part comes to zero, that in contact withthe surface must have this momentum.
Now is this wonderful thought- what if this object is rigid itself! The same must happen to preserve momentum.There is no point in time where the momentum of the system is lesser than the initial of the body.The body must compress- and this means the surface can compress too