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Homework Help: Gravitational potential energy = kinetic energy question

  1. Sep 7, 2010 #1
    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. jcsd
  3. Sep 7, 2010 #2

    Doc Al

    User Avatar

    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
     
  4. Sep 8, 2010 #3
    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
     
  5. Sep 8, 2010 #4

    Doc Al

    User Avatar

    Staff: Mentor

    Exactly! :approve:
     
  6. Sep 9, 2010 #5
    Thank you so much for your help! :shy:
     
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