A bar magnet of mass 
is suspended from the free end of a spring, as illustrated in the figure below.
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 with one pole remaining inside the coil. The other pole remains outside the coil. At time 
, the magnet is oscillating freely as it passes through its equilibrium position. At time 
, the switch in the circuit is closed. The variation with time 
of the vertical displacement y of the magnet is shown in the graph below.
Calculate, to two significant figures, the frequency 
of the oscillating magnet.
.
