A long hollow metallic cylinder is held vertically. A small bar magnet is dropped along its axis. Finally it will fall with (Given: \( g \) is acceleration due to gravity)
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → acceleration \( a = 0 \)
A magnet falling through a metal tube changes the magnetic flux through the rings of the tube, and this induces eddy currents in the tube. By Lenz’s law these currents oppose the motion that produces them, so they push up on the magnet with a force that grows with its speed.
\( ma = mg - F_{\mathrm{magnetic}}(v) \)
As the magnet speeds up, the opposing force grows until it equals the weight. From then on the net force is zero and the magnet falls with a constant terminal velocity. A long cylinder allows this state to be reached, so finally
\( F_{\mathrm{magnetic}} = mg \)
\( a = 0 \)
Note: The question does not say that it asks only for the final, steady state. Before the terminal velocity is reached, the magnet falls with an acceleration between 0 and g, so under that reading Option (2), a < g, is also correct; it is even literally true of a = 0, since 0 is less than g. The word “Finally” points to the terminal state, where a = 0.
Note: This question is from the pre-rationalised NCERT book (2022-23). The topic was removed from the latest NCERT book (2026-27), and it also appears to be out of the CUET 2026 syllabus.
NCERT (Pre-rationalised edition, 2022-23):
Eddy Currents – “… Take two hollow thin cylindrical pipes of equal internal diameters made of aluminium and PVC, respectively. … You will see that the magnet takes much longer time in the case of aluminium pipe. Why is it so? It is due to the eddy currents that are generated in the aluminium pipe which oppose the change in magnetic flux, i.e., the motion of the magnet. The retarding force due to the eddy currents inhibits the motion of the magnet. Such phenomena are referred to as electromagnetic damping.”