# Calculating Velocity Using Power

1. Jul 18, 2013

### thedude36

1. The problem statement, all variables and given/known data
The problem I'm working on gives the total power (100 W) that will be exerted by a man (80 kg) climbing a set of stairs(r = 8mi + 4mj). It would like to know at what constant velocity he would have to move to achieve the given power.

2. Relevant equations
P=dU/dt=F•dr/dt=FV

3. The attempt at a solution

The difficulty I'm having is if he is moving with constant velocity there would be no acceleration and thus no force. So how could there be any power generated? I attempted to find the force the man would have to exert to overcome the force of gravity, which I found to be equivalent to the force of gravity (since they would have to sum to zero). But, when I use this I get a velocity that doesnt make sense (.2849 m/s). Have I got the wrong concept?

2. Jul 18, 2013

### rude man

But there is a force. It's called gravity.

Compute the rate at which he gains potential energy as he climbs the stairs at a rate of 100W. This will equal mg dh/dt.