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Mechanical Energy

  1. Mar 1, 2009 #1
    1. The problem statement, all variables and given/known data

    A 0.13 kg ball moves at a rate of 20.0 m/s at the point where it is 5.00 m above the ground. How much mechanical energy does the ball have with respect to the ground?

    2. Relevant equations

    Ek = 1/2mv^2

    3. The attempt at a solution

    Ek = 1/2mv^2
    Ek = 1/2(0.13kg)(20m/s)^2
    E = 26 J

    The assignment says my answer should be 32.4 J.
    Obviously, I am doing something wrong, but I'm not sure how to incorporate the "with respect to the ground" bit into my equations.

    Somebody offer me some guidance, please?
  2. jcsd
  3. Mar 1, 2009 #2
    I can't comment on WHY this is so.. but apparently.. the answer includes the potential energy of the ball as well

    = mass x gravity x height

    = .13 * 9.80 * 5m = 6.37J

    26 J + 6.37J = 32.4J

    Not really helping but I'm not familiar with mechanical energy...
  4. Mar 1, 2009 #3
    Hmm...that is strange....I really don't like the way that the question is worded.

    But that did help, thank you.
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