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When the pole of a bar magnet is brought close to a magnetic compass, the bar magnet and the compass needle (which is also a magnet) exert a magnetic force on each other. As per Newton's third law of motion, both the forces are equal in magnitude and opposite in direction. However, the compass needle moves, whereas the bar magnet does not move (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.
The compass needle
The bar magnet
Feels a magnetic force of magnitude
F
Feels an equal, opposite force of magnitude
F
Very
small
mass, freely pivoted
Much
larger
mass, resting on the table
a = F/m
is
large
a = F/m
is far too
small
to notice
So the needle
swings round
So the magnet appears
not to move
