Answer:
If the frictional torque is constant, the fan will slow down linearly from its original rotational speed of 90 rpm, equivalent to 1.5 rev/s. Linear slowdown implies that the average rotational speed until a complete stop is half of the top speed. Therefore the fan rotates at an average of 0.75 rotations per second. The time t to stop is therefore:
t=210.75
=28 seconds
