a fan is rotating at 90 rpm. It is then switched off. It stops after 21 revolutions. calculate the time taken by the frictional torque is constant

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

the answer is b

explanation:

since you're a real machine, your ama is always less than your ima, because you can never totally eliminate friction.