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If you have a traditional balance scale, and you put perfectly equal weights on both sides, of course the scale balances the two out, and the two objects being weighed end-up at equal distances from the ground, like a teeter-totter.

Why does the balance equal out, instead of merely remaining wherever one puts it?

It seems to me that if the weights are equal and the distance from the fulcrum is equal, that if I were to exert force to raise one side of the balance (lowering the other), the weights would remain wherever I adjusted them, instead of moving back to the point where the lever is once more parallel. What force causes the movement back to "center"?