[Physics] If magnets exert greater force on heavier objects, why is it possible to separate magnets from very heavy objects

magnetic fields

I was thinking of the question "Do magnets exert more force on heavier (more massive) objects?"

I would think the answer is "yes", because for example if you have a magnet and 2 paperclips, they will both be attracted by a force F to the magnet, and therefore if you tape them together the force will be 2F.

But if this is true, then if you have a very heavy metallic object (e.g. a steel safe), why wouldn't a small magnet get stuck to it with a very high force? If a safe weighs 100kg and a paperclip weighs 1g, that means the force should be 100,000 times as strong. Given that the magnetic attraction force is enough to lift the paperclip, that means you should be able to lift the 100g safe with the same magnet, right?

Well you can't, so I'm wrong. But why am I wrong? I was thinking maybe there is a dropoff in the force somewhere, but then isn't steel a good "conductor" of magnetic force?

Best Answer

The 100 kg safe occupies a large volume of space. There is probably less than 500 g of iron that the magnet influences. Also, the magnetic attraction drops off the further the iron is from the magnet.

Even if the force increases with the amount of iron, a small magnet cannot be used to lift a safe.

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