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Weight and magnetic force

  1. May 9, 2010 #1
    1. The problem statement, all variables and given/known data
    Placed in a cylindrical container of negligible mass, a magnet with weight W is placed on a analytical balance. Another magnet is placed on top of it, hanging in mid-air and is stationary. What is the reading of the analytical balance?

    2. Relevant equations

    3. The attempt at a solution
    By drawing the free-body diagram, the magnet on top will have weight acting downwards and magnetic force upwards. The magnet below will experience weight downwards, normal reaction force up wards and magnetic force downwards. I know the magnetic force must = W since the magnet is not moving. However, why is the reading of the balance = W, and not >W?

  2. jcsd
  3. May 9, 2010 #2
    what do u mean by hanging in mid air? is it only balanced by the repulsive force against weight or it is hanging by some rope or anything?
  4. May 10, 2010 #3
    It is only balanced by the repulsive force against weight.
  5. May 11, 2010 #4


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    You're correct that the magnetic force is W. To answer the question, figure out the force that the balance exerts on the weight+cylinder -- that will be the reading of the balance.

    Hint: your suspicion that the reading is >W is correct.
  6. May 12, 2010 #5
    But, if I'm not wrong, there's newton's third law. The magnetic force will also be exerted on the bottom magnet. So, there's the W of the magnet, and magnetic force W acting downwards as well. Doesn't this mean that total normal reaction exerted by balance will be 2W?
  7. May 12, 2010 #6


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    Yes, 2W is right.
  8. May 13, 2010 #7
    Oh. Okay Thanks :D
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