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Something interesting about magnets and weighing scales

  1. Oct 23, 2007 #1


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    I got hold of a set of kitchen weighing scales which are made of some ferromagnetic plate and my neodymium magnets. So the reading on the weighing scales increases as you bring the magnet near the weighing plate, as expected... they then reach a maximum just before you touch the magnets to the plate. The moment you let the magnets hit the scales, the reading goes down to simply the weight of the magnets (in my case, nearly a fifth of the maximum reading).
    I had always found magnets quite intriguing from a structural mechanics point of view (is magnetic force between 2 magnets/a magnet and a ferromagnetic material an internal force or an external force). It appears that the attraction is treated as an external force until they touch. This all may be very obvious, but comments please!
  2. jcsd
  3. Oct 23, 2007 #2

    Doc Al

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    Staff: Mentor

    Whether the magnetic force is an external force or not depends on your choice of system. If you choose the plate as the system, then the magnetic force is external; but if your system is "magnet + plate", the force is internal.

    As long as the magnet doesn't quite touch the plate, the only force the magnet exerts on the plate (and vice versa) is the magnetic force. Of course you must exert a force on the magnet to prevent it from attaching itself to the plate. Looking at "plate + magnet" as a system, the magnetic force cancels but you are exerting an equivalent restraining force on the magnet.

    Once the magnet sticks to the plate, an additional contact force appears between magnet and plate--they push against each other. You no longer have to exert a restraining force on the magnet. Unless you keep pulling on the magnet, the only net force on the plate is the weight of the magnet. The downward magnetic force on the plate is countered by the upward contact force of the magnet. Of course, if you keep pulling on the magnet with the same force that you exerted before they stuck together, then you wouldn't see any change in scale reading.

    Let me know if this makes sense to you.
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