# Who does work when we walk?

1. May 14, 2013

### deepani

Okay, so I know that static friction does no work since work is defined as:
W = ∫F.ds. The displacement is defined only for the point of contact. So, when one walks, one foot remains on the ground. The static force of friction acts on that leg. Thus the point of application also, lies on that leg. Now, one moves the other leg in the forward direction, thus changing the center of mass of the system (here I consider the whole human body as the system and nothing else). So, we know that only the external forces are capable of changing the center of mass of the system, which, in this case is of course, the static force of friction.
But, what happens when we talk about work? So, is it possible, that although no force does work on a system, there is still some displacement? (I'm not changing my reference frame, initially, object was stationary w.r.t the frame)

Let's say one runs accelerating by a small amount. There is still no work done. But we define : ΔK.E = Work done by conservative, non-conservative and external forces, which in this case is ZERO.
This implies that there should be no change in kinetic which is not true! (i'm only accelerating until the force I apply does not exceed μmg, the max. frictional force)
PLEASE HELP ME WITH THIS.. this question has got me confused. Is there a conceptual blunder, if so please explain. thanks in advance... ANd sorry for such a long write-up..:P

Last edited: May 14, 2013
2. May 14, 2013

### SteamKing

Staff Emeritus
The equation for work does not presuppose that contact between two bodies is necessary for work to be done. If that were true, then airplanes could fly forever without the need to refuel.

For walking, running, etc., a person does work by the simple fact of moving from point A to point B. Where there is contact and there is friction, it takes work to overcome friction.