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When you can't use conservation of energy

  1. Dec 9, 2011 #1
    I have noticed that I try to apply conservation of energy laws to solve various problems, and they don't always work. For example, this problem I'm doing now is a roll of toilet paper that drops to the floor while someone is holding the first sheet of paper.

    Is it the presence of a tension force from the person holding the roll that violates conservation of energy? I keep making this same mistake
  2. jcsd
  3. Dec 9, 2011 #2
    It depends what you have done.
    If you can show us how you attempted to solve the problem it would help.
    The exact wording of the question is also important.
    Questions like this usually say things like "ignore friction" as a clue to being able to use conservation of energy.
    Energy can take a number of forms, and in the question you mention there is kinetic and potential.
    The kinetic energy will be both rotational (the roll spins) and translational (it drops to the ground).
  4. Dec 9, 2011 #3
    Energy is always conserved. But in some cases it is difficult to measure some forms of energy.

    For example:it is rather difficult to measure the energy of the sound made when a roll of paper falls on the floor.
  5. Dec 9, 2011 #4
    Indeed. I could have added that in questions of this type, it's whether or not you can consider mechanical energy to be conserved.
  6. Dec 9, 2011 #5
    Where you draw your 'box' is also important. Energy is conserved in a closed system. But if you let energy cross the boundary of your box (your system), then you can't rely on those equations anymore.
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