Two identical strong magnets are dropped simultaneously by two students into two vertical tubes of the same dimensions (see figure below). One tube is made of copper, and the other is made of plastic.
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The magnet dropped through the plastic tube. Plastic doesn't conduct electric currents with anywhere near the ability of the copper tube. The magnet dropped through the copper tube will induce a voltage around the loop as per Faraday's law of induction. Such a voltage creates a current through the tube, and that current makes this copper tube act as an electromagnet. This electromagnet will apply a force to the magnet which opposes the gravitational force on the magnet. How do I know it will oppose? Because of conservation of energy. In order for the current dissipating heat in the copper tube (due to the voltage and current combination), the magnetic force MUST do negative work on the falling magnet, such that the magnet does work on the tube as it falls.
