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Does the potential energy of a spring do work on the both

  1. Feb 22, 2015 #1
    Suppose we have two boxes, A and B with different masses, the spring is fixed to block A.

    Then we take block B and press it against block A so that the spring compresses.

    Afterwards, we let go of both boxes.

    If we were to model this using the conservation of energy, then it is known that

    (KE of A)1 + (KE of B)1 + (PE of spring)1= (KE of A)2 + (KE of B)2 + (PE of spring)2

    0 + 0 + (PE of spring)1= (KE of A)2 + (KE of B)2 + 0

    But one thing is confusing me. That is, the potential energy of the spring.

    I know that upon release, it will do positive work on block B. But won't it also do positive work on block A?

    Doing an experiment, when I push two masses together with a spring,with the spring attached to one box (a pen spring) both boxes go flying both ways.

    Surely this affects the potential energy?

    So how do I account for this potential energy?Should it be double, or what?
    Last edited: Feb 22, 2015
  2. jcsd
  3. Feb 22, 2015 #2


    Staff: Mentor

    Yes, assuming that both blocks are free to move, it will do work on both.

    Based on your personal experience using springs to push things around do you think that a spring will push a block faster if the other side of the spring is pushing on a fixed wall or if the other side of the spring is pushing on another free block? (Assuming equal spring compression). What does that tell you about where the energy goes?
  4. Feb 22, 2015 #3
    If the spring is attached to a wall, then it can only decompress one way. Whereas if its between two blocks, it can do decompress both ways. So the block-wall spring should push the block faster.

    I guess this means in the block-block spring, some of the energy is used to push both boxes. Actually, come to think of it, in both cases it transfers the same amount of energy. The only difference is the wall can't move., so its velocity is 0.

    So then we still use the same formula for potential energy of the spring, 1/2kx^2. Right?
  5. Feb 22, 2015 #4


    Staff: Mentor

    Right, the PE is the same, and just not all of it goes into block B upon release.
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