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A circular disc is rotating about a central axis perpendicular to its diameter with a speed of 5rad/s. If a constant force of 2N is applied tangentially on it, in how much time will it come to a stop? Radius of disc = 1m & mass = 1kg.

Correct Answer: 1.25s
Let r be the radius, I be the moment of inertia, a be the angular acceleration & F be the force applied. Moment of inertia of disc about given axis = MR2/2 = 0.5 kgm2. Torque = r X F = 2Nm. But, torque is also = Ia. ∴ a = 2/0.5 = 4 rads2. Using, w = w0 + a*dt, we get: 0 = 5 – 4dt, where dt is the time interval. ∴ dt = 5/4 = 1.25s.

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