# Gravitational potential energy = kinetic energy question

1. Sep 7, 2010

### vf_one

Hi

I'm just going over some revision questions and there's this one question where I'm not sure if my working is correct (we don't get given the answers).

1. The problem statement, all variables and given/known data
A bobsled is moving at 3m/s as it passes a timing location. It then descends through a height of 3m before passing the next timing location. Assuming that friction and air resistance can be neglected, what is the speed of the bobsled as it passes the second timing location

2. The attempt at a solution

Ep=Ek
mgh=1/2mv2
v2=2gh

v=7.668m/s

Speed of bobsled passing second timing location = 3 + 7.668 = 10m/s

I'm not too sure if it is right to add 'v' with the initial speed of 3m/s. If anyone can clarify this for me that would be great.

thanks!

2. Sep 7, 2010

### Staff: Mentor

No, it's not correct.

Instead of using Ep = Ek, use the more general version of mechanical energy conservation:
Ep1 + Ek1 = Ep2 + Ek2

3. Sep 8, 2010

### vf_one

Hey Doc_Al

Thanks for your help

So would this be correct then?

Ep1+Ek1 = Ek2 + Ep2

1/2 x 32 + 9.8 x 3 = 1/2 x v2
v = 8.2m/s

4. Sep 8, 2010

Exactly!

5. Sep 9, 2010

### vf_one

Thank you so much for your help! :shy:

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