17.2 Damped Oscillations

A bar magnet of mass 250 g is suspended from the free end of a spring, as illustrated in Fig. 3.1.

Fig. 3.1

The magnet hangs so that one pole is near the centre of a coil of wire.
The coil is connected in series with a resistor and a switch. The switch is open.

The magnet is displaced vertically and then allowed to oscillate.

At time t = 0, the magnet is oscillating freely. At time t = 6.0 s, the switch in the circuit is closed.

The variation with time t of the vertical displacement y of the magnet is shown in Fig. 3.2.

Fig. 3.2

For the oscillating magnet, use data from Fig. 3.2 to determine, to two significant figures:

f = ...................................... Hz [2]

energy = ....................................... J [3]

When the switch is closed, the oscillations are damped.

Explain, with reference to Fig. 3.2, whether this damping is light, critical or heavy.