Question

A disc spinning on its axis at 20 rad/s will undergo precession when a torque 100 N-m is applied about an axis normal to it at an angular speed, if mass moment of inertia of the disc is the 1 kg-m^2

(a) 2 rad/s

(b) 5 rad/s

(c) 10 rad/s

(d) 20 rad/s

I got this question at a job interview.

Query is from Precessional Angular Motion in section Gyroscopic Couple and Precessional Motion of Machine Dynamics

Answer 1

The correct answer is (b) 5 rad/s

Easiest explanation: Gyroscopic couple acting on the disc,

C = I. ω. ωp

100 = 1 . 20. ωp

ωp = 100/1.20

                          = 5 rad/s