First slide

Torque

Question

A disc of mass 4 kg and radius 50 cm is rotating with angular velocity 30 rad/s about a spindle passing through its centre and perpendicular to the plane of the disc. A constant braking torque is applied on the disc and the disc stops spinning in 10 second. Then the magnitude of the braking torque is

Easy

A

4 N-m
B

2.5 N-m
C

1.5 N-m
D

2 N-m

Solution

Moment of inertia $I = \frac{1}{2} \times 4 \times {(0.5)}^{2} K g - m^{2} = 0.5 k g - m^{2}$
using $ω = ω_{0} - α t \Rightarrow 0 = 30 - α \times 10 \Rightarrow α = 3 r a d / s^{2}$
$∴$ Braking torque $τ = I α = 0.5 \times 3 N - m = 1.5 N - m$

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