# How work is done

1. Nov 21, 2013

### Miraj Kayastha

If we start walking from rest, is work done while walking?
I think work is done because kinetic energy increases and if we come to rest the work done is again 0 at last.

Is this true?

2. Nov 21, 2013

### Staff: Mentor

Work is done on a number of levels when we walk. We leave footprints (that requires expenditure of energy) and particles of rubber get scuffed off (ditto). As we take each step we lift our weight on one side by a cm or two as we straighten our leg on that side from bent to fully straight. That's work, lifting your whole weight against gravity by a cm or two every pair of steps (and in the process we lift the weight of each lower leg by more than this).

If you were to walk half a mile, the energy you return to the earth as you finally come to a standstill is a miniscule fraction of the energy you expended doing those myriad small repetitive work tasks involved in locomotion.

3. Nov 21, 2013

### Miraj Kayastha

So do we gain kinetic energy?
And this may sound weird but when we walk we displace air. If so do we do work on air?

4. Nov 21, 2013

### nil1996

Work is force times displacement.
So anything you displace you have to do work on it :)

5. Nov 21, 2013

### Miraj Kayastha

then how do we calculate the work done on air?

6. Nov 21, 2013

### Staff: Mentor

This is a do-able calculation, but it involves solution of complicated fluid mechanics equations, including viscous terms.

Chet