Gravitational potential energy = kinetic energy question

[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).

Homework Statement

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!

 

[Doc Al]

I'm not too sure if it is right to add 'v' with the initial speed of 3m/s.

No, it's not correct.

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

 

[vf_one]

Hey Doc_Al

Thanks for your help

So would this be correct then?

E_p1+E_k1 = E_k2 + E_p2

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

 

[Doc Al]

So would this be correct then?

E_p1+E_k1 = E_k2 + E_p2

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

Exactly!

 

[vf_one]

Thank you so much for your help! :shy: