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When the pole of a bar magnet is brought near a magnetic compass, both feel equal and opposite magnetic forces (Newton's third law). Yet the compass needle moves while the bar magnet does not (Fig. 6.42). Explain why.
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Though the forces are equal, the compass needle has a very small mass, so the same force gives it a large acceleration (a = F/m) and it turns easily. The bar magnet is much heavier, so the same force gives it a tiny acceleration that is not noticed.
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