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Homework Help: Cons. of Energy, momentum ,and impact.

  1. Mar 19, 2012 #1
    I attached the problem statement only with PART of the solutions.
    He is using the conservation of energy to find the velocity of the anvil.
    T=Kinetic energy
    V sub e=potential energy of the spring
    V sub g=potential energy of gravity

    The part I am not understand is how ΔV sub e = 1/2(2.8*10^6)(0.024)^2+29.4(10^3)(0.024) J

    Why do they have the 29.4(10^3)(0.024) there?? That's potential energy of gravity. My professor said the answer was right and that he has checked over it, but he takes a long time to respond so I'm asking it on here.

    Attached Files:

  2. jcsd
  3. Mar 20, 2012 #2
    That's because the springs have a massive 3000kg anvil on it. So you have to account for the force of that on it and the distance it has moved as well.

    So when that spring is released, it's moving itself as well as the anvil, to account for that extra energy you have to include the anvil.
  4. Mar 20, 2012 #3


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    Staff: Mentor

    The problem is, the given solution first includes the gravitational PE with the spring PE, then subtracts it again under the separate guise of gravitational PE. Thus the gravitational PE is essentially being ignored entirely for the anvil/spring compression.

    I'd call this highly suspicious behavior! It looks like a fudge to make the solution match the given answer. Perhaps whoever provided the book answer simply forgot to include gravitational PE when they solved it, and now the only way to match the "correct" answer is to recreate their error.
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