If you had two scales, one made of metal one made of something like plastic (not magnetic), and weighed the same magnet on both scales would it read heavier on the metal scale rather than the plastic one?
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1 Answer
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It depends.
If it's a scale like this:
Then it would depend on the mass distribution of the scale, assuming it was all magnetic and same material.
If it's a scale like this:
then assuming that only the 2D plate is magnetic and the rest is not(which may not be true), then no.
The plate and magnet are part of a system.
If they are attracting: If the magnetic force makes the plate move upwards, then since the magnet rests on that plate, it must move up too. So the center of mass of the system moves. But the magnetic force is an internal force, and thus cannot affect the COM.
If they are repelling each other:
At equilibrium, the magnet must be at rest. So the magnetic force must counteract the weight of the magnet, and thus be equal. But the magnetic force on the magnet is the same as that on the plate, so the magnetic force on the plate equals the weight of the magnet. But the magnet's actual weight does not contribute because the magnet is no longer on the plate.