Gravitational potential energy = kinetic energy question

AI Thread Summary
The discussion revolves around calculating the speed of a bobsled after descending a height of 3m, starting from an initial speed of 3m/s. The initial approach incorrectly added the initial speed to the calculated speed from gravitational potential energy. Instead, the correct method involves applying the conservation of mechanical energy, leading to the equation Ep1 + Ek1 = Ep2 + Ek2. After recalculating using this formula, the correct final speed of the bobsled is determined to be 8.2m/s. The clarification on using energy conservation principles resolves the initial confusion.
vf_one
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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!
 
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vf_one said:
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
 
Hey Doc_Al

Thanks for your help :biggrin:

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
 
vf_one said:
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
Exactly! :approve:
 
Thank you so much for your help! :shy:
 
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