Since the rods experience a changing magnetic flux as the magnets make their descent, emf is induced in the rods. The resulting eddy current creates a magnetic field that opposes the descent of the magnets, resulting in a retardation force on the magnets.

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Assuming all four magnets are identical, then the induced emf in all the four rods should be exactly the same. However, the induced current is different due to the different resistivities of the four rods. Copper, with the lowest resistivity, has the largest induced current and thus experiences the largest retardation force. Plastic is basically an insulator and has zero induced current (despite the same induced emf). It thus suffers practically no retardation force.

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