Question

A circular disc is rotating about its own axis. An external opposing torque of magnitude $0.02Nm$ is applied on the disc, due to which it comes to rest in $5$ seconds. The initial angular momentum of the disc is:

• A. 0.1 $kg{m}^2{s}^{-1}$
• B. 0.04 $kg{m}^2{s}^{-1}$
• C. 0.025 $kg{m}^2{s}^{-1}$
• D. 0.01 $kg{m}^2{s}^{-1}$

Answer 1

The correct option is A 0.1 $kg{m}^2{s}^{-1}$

$\overline{C}=0.02N/m$
$t=5sec$

torque is applied for $5sec$ and torque is rate of change of angular momentum so,
angular momentum lost due to decelaration $=\overline{C}\times △t$
$=\left(0.02\right)\times 5$
$=0.1$
change in angular momentum $=$ final angular momentum $-$ initial angular momentum
$0.1=0-$ Initial angular momentum
$\therefore$ Initial angular momentum = $0.1kg{m}^2/s$