# Homework Help: Crane power homework question

1. Dec 3, 2007

### alex7298

1. The problem statement, all variables and given/known data
2 problems.
1: A man lifts a 91kg person straight up a distance of 1.2m in 0.51s. What power is used to do this?

2: 3.00x10^2kg piano is lifted at a steady velocity from ground to a height of 10.0m. The crane doing the lifting has a steady power of 4.00x10^2W. What time does it take to do this?

2. Relevant equations
Both equations should be able to use P=Fd/t

3. The attempt at a solution
With both problems, the book states constant velocity so the F=ma=0, which doesnt make sense because that would make the answer 0.
Is there another way of finding force that i am forgetting?
Thanks

2. Dec 3, 2007

### PhanthomJay

That makes the net force and the net work = 0. But you're not looking for the net, you're looking for the work done by just one of the forces against gravity.

3. Dec 3, 2007

### alex7298

would I use GPE=mgh for the work?
so for problem 1: work=(91kg)(9.8)(1.2)=1070.16J===> P=1070.167J/0.51s=2098.366W?

4. Dec 4, 2007

### PhanthomJay

Yes, correct, as long as you understand why you are equating the work done by the man to the potential energy change of the person.