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Homework Help: PE & PE Energy

  1. Aug 11, 2012 #1
    1. The problem statement, all variables and given/known data
    Using the formulae for potential energy and kinetic energy, find the speed that a tennis ball hits the ground when dropped from a height of 20m.

    With reference to the principle of conservation of energy, explain why you could use these equations and what assumptions you have made.

    2. Relevant equations
    KE = 1/2mv^2
    PE = mgh

    3. The attempt at a solution
    The first part I think I'm ok with. The tennis ball is dropped therefore no KE only PE.
    PE = 9.8 x 20 = 196 Joules.

    It is the second part that I don't really understand. What is it asking me? Can somebody maybe re-frase this part of the question. Thank you.
  2. jcsd
  3. Aug 11, 2012 #2
    sorry. I forgot to turn that PE into KE by making 196 = 1/2v^2
    This would give 19.79m/s
  4. Aug 11, 2012 #3


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    Staff: Mentor

    You can't just drop mass from your equation because the ball's mass is not stated. :yuck:
    Keep mass where it belongs, but represent it by the symbol m.
    PE = m x 9.8 x 20 = 196m Joules
  5. Aug 11, 2012 #4


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    Science Advisor
    Homework Helper
    Gold Member

    Also best to avoid plugging in the number until the end.

    PE = mgh
    KE = 0.5mv2

    0.5mv2 = mgh

    Mass cancels.

    Rearrange to give

    v = SQRT(2gh)

    Then put the numbers in.

    As for part 2....

    You need to explain why the KE the ball has when it hits the ground will be equal to the PE it had when dropped. What assumptions does that statement rely on?

    Hint: Why would it be more reasonable to make that ssumption on the moon than on earth?

    Make sure your answer mentions conservation of energy.
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