# Homework Help: Find MASS of a runner when work, power, and velocity are given?

1. Oct 25, 2009

### kalisious

1. The problem statement, all variables and given/known data
While running in a cross country meet, a member of the team dissipates about 0.35J of mechanical energy per step per kg of her body mass. If she develops a power of 75W during the race and runs at a speed of 5m/s, find her mass.
PS: Consider one [of her] running steps to be 1m.

2. Relevant equations
I am unsure what equation to use to find mass. I know the equations to find power and work, but in relation to finding mass I am unsure.

3. The attempt at a solution
I cannot attempt a solution with no clue what equation to use to get me started.
I am able to do the work if I have proper equations.
Thanks!

2. Oct 25, 2009

### Delphi51

Looks like the 75 Watts goes into making those steps.
P = W/t so you need to figure out the work done stepping over some convenient time. Maybe you could do it for one step?

3. Oct 25, 2009

### kalisious

I am not sure what equation is used to find the mass though?

4. Oct 25, 2009

### Delphi51

It says the energy used is 0.35 J per kg. So the energy used is 0.35*m.
If you put that in the equation, you'll be able to solve for m.