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Question

Physics Question on momentum

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

A

0.1 kgm2s−10.1\,kgm^2s^{-1}

B

0.04 kgm2s−10.04\,kgm^2s^{-1}

C

0.025 kgm2s−10.025\,kgm^2s^{-1}

D

0.01 kgm2s−10.01\,kgm^2s^{-1}

Answer

0.1 kgm2s−10.1\,kgm^2s^{-1}

Explanation

Solution

Rate of change of angular momentum is equal to the torque:
τ=dLdt\tau=\frac {dL}{dt}

τ=(Lf−Li)△t\tau= \frac {(L_f-L_i)}{△ t}
Where,
LfL_f= Finial angular momentum
LiL_i= Initial angular momentum.
As external force is mentioned, therefore we can say the torque is retarding which will be in negative.
−0.02 Nm=0−Li5-0.02\ Nm=\frac {0-Li}{5}

0.02 Nm=Li−050.02\ Nm=\frac {Li-0}{5}
Li=0.10 kgm2s−1Li=0.10\ kgm^2s^{-1}

So, the correct option is (A): 0.10 kgm2s−10.10\ kgm^2s^{-1}.