A metal bar, pivoted at one end, oscillates freely in the absence of a magnetic field. But when it oscillates between the poles of a magnet, its oscillations are quickly damped. Why? (Such magnetic damping is used in a number of practical devices.)

When the metal bar oscillates between the poles of a magnet, it experience a change in the magnetic flux (no of magnetic field lines passing through the metal bar) as it enter or leaves the magnetic field of the poles. As we know that the change in magnetic field induces the electric current in the metal bar (conductor). By considering the lenz law which states that the direction of induced current in the conductor will be such that as to oppose the initial magnetic field that is producing it. This opposing force acting on the metal bar will damp the oscillations of the bar between the poles of a magnet.