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# Food for thought: Solve this Physics problem

THINK:....a bar magnet is dropped without any initial velocity in a very long(but not considerable to earth's radius) hollow copper tube which is fixed to the ground of earth..the bar magnet is falling freely under gravity .......now my question:::::Can the accelleration of the bar magnet be zero...??............ think logically to crack it.....happy problem solving....thanks in advance for those who join this.....:)

Note by Raja Metronetizen
3 years, 5 months ago

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Yeah, it can be zero. By lenz law an upward force would act to oppose the change in magnetic field. As the speed of bar magnet increases, the change in magnetic field would increase and so does the force. A time would come when the upward force would balance downward force of gravity and bar magnet would fall with terminal velocity. Am I correct? Can someone help me finding the relation between upward force and rate of change in magnetic field?

Thanks for the question :) · 3 years, 5 months ago

you are exactly right in your description ......and you could also consider the matter of eddy current for this description.....then you deciphered the problem....:) · 3 years, 5 months ago

I now recall what happens exactly. The changing magnetic field produces eddy currents in the tube. The eddy current constitutes a magnetic dipole which repels the bar magnet. Right? · 3 years, 5 months ago

yea...right...!! · 3 years, 5 months ago