JEE Mains · Physics · STD 11 - 6. system of particles and rotational motion

A circular disk of radius R meter and mass M kg is rotating around the axis perpendicular to the disk. An external torque is applied to the disk such that \(\theta(t)=5 t^2-8 t\), where \(\theta(t)\) is the angular position of the rotating disc as a function of time \(t\). How much power is delivered by the applied torque, when \(t=2 s\) ?

A \(72 MR ^2\)
B \(8 MR ^2\)
C \(108 MR ^2\)
D \(60 MR ^2\)

Verified Solution

Answer & Solution

Correct Answer

(D) \(60 MR ^2\)

Step-by-step Solution

Detailed explanation

\(\theta =5 t ^2-8 t \) \( \omega =\frac{ d \theta}{ dt }=10 t -8 \) \( \alpha =\frac{ d \omega}{ dt }=10 \) \( \therefore p =\tau \omega \) \( =( I \alpha) \omega \) \( =\left(\frac{ mR ^2}{2}\right) \alpha \omega \) \( =\left(\frac{ mR ^2}{2}\right)(10)(10\) Put \(t =2\)…

Apply the relevant identity from the chapter and substitute the values established in the previous step, keeping the original constraint in view.

Use the simplified expression to isolate the unknown, then plug the result back into the question setup to confirm the working is internally consistent.

Cross-check against a boundary or limiting case. Both the dimensional balance and the qualitative behaviour at the extreme should agree with the candidate answer.

Combine the steps above to identify the correct option. The remaining choices each fail at least one of the consistency, dimensional, or boundary checks.

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