Discussion
An iron ring is held horizontally and a bar magnet is dropped gently through the ring with its length coinciding with the axis of the ring. The acceleration of the freely falling magnet through the ring
(g = acceleration due to gravity)
Answer: A
The acceleration of the magnet is less than g.
As the magnet falls through the ring, the magnetic flux through the ring changes, inducing currents (eddy currents) in the conducting iron ring. By Lenz's law these currents oppose the change that causes them: the ring repels the approaching magnet and attracts the receding one, so in both phases there is an upward force on the magnet and its acceleration is less than g.
It would equal g only if no current could be induced, and it cannot exceed g because the induced force always opposes the motion.
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