Both experience equal magnetic forces by Newton’s third law. The light compass needle pivots freely with negligible friction, whereas the much heavier bar magnet has high inertia and is held stationary by static friction with the surface.
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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According to Newton’s third law, the mutual magnetic forces are equal and opposite. However, the tiny compass needle has very small mass and is mounted on a near frictionless pivot, allowing the force to easily turn it. In contrast, the bar magnet has a much larger mass, higher inertia and experiences significant friction from the resting tabletop, which prevents it from moving.
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