A slender uniform rod of mass \(M\) and length \(l\) is pivoted at one end so that it can rotate in a vertical plane (see figure). There is negligible friction at the pivot. The free end is held vertically above the pivot and then released. The angular acceleration of the rod when it makes an angle \(\theta\) with the vertical is

A \(\frac{3 g}{2 l} \sin \theta\)
B \(\frac{2 g}{3 l} \sin \theta\)
C \(\frac{3 g}{2 l} \cos \theta\)
D \(\frac{2 g}{2 l} \sin \theta\)

Verified Solution

Answer & Solution

Correct Answer

(A) \(\frac{3 g}{2 l} \sin \theta\)

Step-by-step Solution

Detailed explanation

Torque at angle \(\theta\) \(\tau=M g \sin \theta\) \(\tau=l \alpha\) \(l \alpha=M g \sin \theta \frac{l}{2}\) \(l=\frac{M l^{2}}{3}\) \(\frac{M l^{2}}{3} . \alpha=M g \sin \theta \frac{l}{2}\) \(\frac{l \alpha}{3}=g \frac{\sin \theta}{2}\) \(\alpha=\frac{3 g \sin \theta}{2 l}\)

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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