A bar magnet is made to fall through a long vertical copper tube the speed of the magnet as a function of time is best represented by
Answers
Initially, due to acceleration of the magnet the rate of changing of linked flux (−dϕ/dt is itself changing and hence the induced emf, and the force due to it, is increasing (changing). Ultimately at a sufficiently high velocity, the force due to induced current will be equal to the weight. When F=mg, then the net force and hence acceleration is zero, but the flux is still changing if it has a non zero velocity. After this, since velocity is constant and no acceleration exists, the induced emf will be constant because −dϕ/dt is constant due to constant velocity. This means the forces will no longer vary and the magnet will continue with same speed.
Your analysis assumed the force due to induced emf to still change after F=mg but that is incorrect as constant velocity will cause constant induced force, in the perfectly long tube's case.