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Magnetic force

  1. Oct 17, 2007 #1
    If a magnet is used to levitate another magnet (using the force of repulsion) can it be considered as doing "work" in opposing the force of gravity? If yes then why does the energy required to do this "work" not diminish over a period of time?
  2. jcsd
  3. Oct 17, 2007 #2
    Your scenario is not so clear. I just guess. If a magnet can repulse another magnet (or levitate), it creates some work. But the work comes from you when you try to place the 'another' magnet close enough to the first magnet (in case the first is a permanent one). If the first magnet is electrical, then the work comes from the electricity energy. It is very much like you try to lift a heavy bar up and let it drop, or you press a spring then release it.
  4. Oct 17, 2007 #3
    If the magnets are stationary, then there is no work being done.
  5. Oct 17, 2007 #4
    Yes, I understand that since there is no displacement no work is being done, but there is obvisously some energy being used to keep the magnet levitated and opposing gravity
  6. Oct 17, 2007 #5
    A book resting on a table is being held against gravity too, but no energy is being expended to keep it there.
  7. Oct 18, 2007 #6
    Yes, but if you were to take the book and hold it up, you would be using energy to do so, not only to lift the book but also to hold it in a fixed position above the table
  8. Oct 18, 2007 #7


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    Staff: Mentor

    The chemical energy that your body uses to do everyday things is different from the mechanical work that goes on outside your body. From that standpoint, your body is very inefficient. Your body truly is doing no work on that book. Look at the definition of work! w=fd. If there is no distance (no motion), there is no work.
  9. Oct 18, 2007 #8


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    Staff: Mentor

    The key phrase here is "on that book". As you hold the book up, there is activity going on inside your arm muscles, with muscle fibers ratcheting back and forth. Those fibers are doing work which consumes chemical energy and gets transformed into heat, and makes you get tired eventually, but none of that energy gets transferred to the book.
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