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The sum of elastic and gravitational energy

  1. Aug 8, 2015 #1
    1. The problem statement, all variables and given/known data
    1. What is the gravitational energy (relative to the unstretched surface of the trampoline) of the 20kg ball at its apex 2.0m above the trampoline
    2. What is the kinetic energy of the ball just before impacting the trampoline
    3. At maximum stretch at the bottom of the motion, what is the sum of elastic and gravitational energy of the ball?
    4. What conclusions can you draw for the answers above

    2. Relevant equations
    I think I've got them all right except 3 in which Im not sure how to approach it

    3. The attempt at a solution
    1. Eg = mgh = 392J
    sig digs make it 400J

    2. All energy is transferred into kinetic energy therefore
    Ek = 400J

    3. ???? Like what I've got is mg(2 +x) but like I'm not sure if I'm approaching it correctly

    4. Through the answers above, I can conclude that energy is always conserved through the law of conservation of energy
  2. jcsd
  3. Aug 8, 2015 #2


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    Staff: Mentor

    I think there is some part of the problem statement missing.
    (3) can be solved with energy conservation alone.
  4. Aug 8, 2015 #3
    What do you mean?
  5. Aug 8, 2015 #4
    Hint : What is the KE of the object at maximum stretch ? What was it initially ? What does the work - energy theorem state ?
  6. Aug 8, 2015 #5


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    That's the loss in PE, but that's not what you are asked for.
    Bear in mind that you are taking the zero PE level as the unstretched trampoline height. In that frame, you started with 400J. Where has all that gone when at bottom of bounce?
    No, you cannot conclude that. You assumed that in order to answer the questions.
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