Question

A small magnet $M$ is allowed to fall through a fixed horizontal conducting ring $R$. Let $g$ be the acceleration due to gravity. The acceleration of $M$ will be

• A. $<g$ when it is above R and moving toward R.
• B. $>g$ when it is above R and moving toward R.
• C. $<g$ when it is below R and moving away from R.
• D. $>g$ when it is below R and moving away from R.

Answer 1

Answer: (A), (C)

$<g$ when it is below R and moving away from R.

When north pole of a magnet M starts moving towards a ring R parallel to its axis, the motion will be opposed according to lenz's law and hence current will be induced due to change in flux such that it will repel the motion of magnet. Hence the magnet experiences an acceleration $a_r$ due to ring. Thus while moving towards ring net acceleration will be less than $g$.

Similarly when magnet moves away from the ring, the magnet experiences attractive force and hence an upward acceleration. Thus net acceleration will be less than $g$ when magnet is moving away from the ring.