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Donkey for a cart and the carrot for a magnet

  1. Sep 19, 2005 #1
    Me and my buddy had a discussion the other day.
    Imagine the scenario with the donkey that has a danglin carrot in front of him. If we swap the donkey for a cart and the carrot for a magnet, what would happen?
    My buddy was pretty sure that the cart would move, and I thought that it wouldn't. So who is right?
  2. jcsd
  3. Sep 19, 2005 #2
    If the conditions were such that the two objects were magnetically attracted, they would not move (roll) if the magnet is rigidly attached to the cart by some kind of overhang. If the magnet is not attached to the cart in any way then there will be some attraction.
  4. Sep 19, 2005 #3
    yeah, there would be an overhang. The magnet and cart would be one, but there's a gap between them. My friend is still convinced that the cart will roll anyway. Can anyone please explain why it won' t ?
  5. Sep 19, 2005 #4
    What the cart is made of is important, as to whether you get paramagnetic or ferromagnetic effects.

    It depends whether you mean 'will the cart will move', or 'will there be a magnetic force'. Also remember there will always be very weak diamagnetic effects, because all substances are diamagnetic.

    If you're talking about a four-wheeled horse of Troy to substitute for your donkey, and a carrot-shaped magnet, the cart won't move. But if the cart is an iron Matchbox car, and your magnet is very strong, it will...
  6. Sep 19, 2005 #5
    If the magnet and cart are rigidly attached, then there's something holding them apart. So all that will happen is the stuff between the magnet and the cart will get squashed. If it's rigid, it can't get squashed. So nothing moves.
  7. Sep 19, 2005 #6
    If it were true that you could get something to move as your friend proposes, you could weld a magnet on a stick to your car, and you wouldn't need an engine.
  8. Sep 19, 2005 #7


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    Simple answer : Newton's Third Law. The force pulling the cart forward to the magnet = force pulling the magnet bacwards to the cart. Since the magnet is rigidly attached to the cart, this force is transferred to the cart. The total force on the whole system is thus, ZERO. Momentum is conserved - nothing happens.
  9. Sep 19, 2005 #8
    Thanx alot all. I guess gokul's answer will put an end to our discussion, and my friend's arguments :smile:
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