Question

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.

(a) 0.257s

(b) 5s

(c) 1.25s

(d) 2.5s

The question was asked during an interview.

Query is from Kinematics of Rotational Motion about a Fixed Axis topic in section System of Particles and Rotational Motion of Physics – Class 11

Answer 1

Correct option is (c) 1.25s

For explanation: 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 = MR^2/2 = 0.5 kgm^2.                                                           Torque = r X F = 2Nm.

But, torque is also = Ia.                                                                                                                                                        ∴ a = 2/0.5 = 4 rads^2.                                                                                                                                                          Using, w = w0 + a*dt, we get:                                                                                                                                              0 = 5 – 4dt, where dt is the time interval.

∴ dt = 5/4 = 1.25s.