# Homework Help: Bob throwing rock - work done

1. Oct 31, 2007

### aligass2004

1. The problem statement, all variables and given/known data

Bob can throw a 620g rock with a speed of 30m/s. He moves his hand forward .8m while doing so.
a.) How much work does Bob do on the rock?
b.) How much force, assumed to be constant, does Bob apply to the rock?
c.) What is Bob's maximum power output as he throws the rock?

2. Relevant equations

W = Fd
P = W/t

3. The attempt at a solution

For part a, I tried using W = Fd. I assumed F would be mg, but I think that the accelerating has to be factored in there somewhere as well. I got 4.861, but it was wrong.

2. Oct 31, 2007

### hotcommodity

You're not given the acceleration of the rock, however you do know its mass and its final speed so to speak. Assuming the rock accelerates from rest, we can use the work-energy theorem to find the work done on the rock. So we have:

$$W_{TOT} = \Delta KE$$

Does that help?

3. Nov 1, 2007

### aligass2004

I know that for K, I would substitute 1/2m(v^2), but I don't know how to find E.

4. Nov 1, 2007

### hotcommodity

I'm not sure what you mean by "E." You're asked to find work, force, and power. What are you stuck on exactly?

5. Nov 1, 2007

### aligass2004

Oh, oh....KE meant change in kinetic energy. I thought it meant kinetic energy times energy.....I don't know why I thought that though.