Three identical magnets are stuck to a magnetic stainless steel disc as shown in the photograph below. (The magnets are the orange rectangles.)
The disc will now be rotated, slowly at first, and then at a faster and faster angular speed.
In what order will the magnets fly off the disc?
- (a) The outer magnet will come off, then the middle magnet, then the inner magnet.
- (b) The inner magnet, then the middle one, then the outer one.
- (c) They will all fly off at the same time.
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The answer is (a): the outer magnet will fly off the spinning disc first, followed by the middle magnet and the inner magnet in that order, as can be seen in this video.
The centripetal force required to hold any object in a circular orbit is greater for larger radii. In fact, the force is proportional to the square of the angular speed and goes linearly with the radius for a constant angular speed. Therefore, at any angular speed, the force is greater for larger radii. If the magnets have the same strength, the one at the larger radius will therefore fly off first.