A bar magnet of mass 250 g is suspended from the free end of a spring, hanging so that one pole is near the centre of a coil of wire; the coil is connected in series with a resistor and an open switch. The magnet is displaced vertically and then allowed to oscillate. At time t = 0 the magnet is oscillating freely; at t = 6.0 s the switch is closed.

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