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Homework Help: Momentum, work, and energy

  1. Nov 14, 2013 #1
    1. The problem statement, all variables and given/known data
    A ball falls from rest from an initial height of 5m above the ground. At what height above ground level will the ball be moving at 5.2 m/s?

    2. Relevant equations
    gravitational energy = mgh
    kenetic energy=(1/2)mv^2

    3. The attempt at a solution
    I tried solving for both types of energy but I do not have mass so I am not sure how to solve. Please help!
  2. jcsd
  3. Nov 14, 2013 #2
    What you want to do is try using the conservation of energy theorem:
    Ki + Pi = Kf + Pf
    It starts from rest so Ki = 0, we want to find hf, which is in Pf, so solve for that

    In the end your equations should work out to be
    m*g*hi - (1/2) * m *vf2 = mghf
    Use algebra to solve for hf. You can see that all the masses cancel, so you don't need them, and the velocity you have is vf! Let me know if it works.
  4. Nov 14, 2013 #3
    Thank you! It worked!
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