Question

When a bar magnet falls through a long hollow metal cylinder fixed with its axis vertical, the final acceleration of the magnet is?

• A. Equal to $0$
• B. Less than $g$
• C. Equal to $g$
• D. Equal to $g$ in the beginning and then more than $g$

Answer 1

The correct option is A

Equal to $0$

1. When a bar magnet falls vertically through the hollow section of a metal cylindrical tube, the magnetic flux associated with the tube changes, causing eddy currents to form in the tube's body. Eddy currents, according to Lenz's law, counter the magnetic bar's descent.
2. As a result, the bar magnet is subjected to a retarding force. The retarding force is proportional to the magnet's velocity.
3. When the bar magnet's velocity rises, the retarding force rises as well, until the retarding force equals the bar magnet's weight.
4. The magnet then achieves a steady terminal velocity. For example, a bar magnet will eventually fall with zero acceleration.
5. As a result, when a bar magnet passes through a long hollow metal cylinder with its axis fixed vertically, the magnet's final acceleration is zero.