JEE Mains · Physics · STD 11 - 3.2 motion in plane

A particle is released on a vertical smooth semicircular track from point \(X\) so that \(OX\) makes angle \(\theta \) from the vertical ( see figure). The normal reaction of the track on the particle vanishes at point \(Y\) where \(OY\) makes angle \(\phi \) with the horizontal. Then

A \(\sin \,\phi = \,\cos \,\phi \)
B \(\sin \,\phi = \frac{1}{2}\,\cos \,\theta \)
C \(\sin \,\phi = \frac{2}{3}\,\cos \,\theta \)
D \(\sin \,\phi = \frac{3}{4}\,\cos \,\theta \)

Verified Solution

Answer & Solution

Correct Answer

(C) \(\sin \,\phi = \frac{2}{3}\,\cos \,\theta \)

Step-by-step Solution

Detailed explanation

\(\frac{\mathrm{mv}^{2}}{\mathrm{r}}=\mathrm{mg} \sin \phi\) \(...(i)\) \(\mathrm{mg} \mathrm{r} \cos \theta=\frac{1}{2} \mathrm{mv}^{2}+\mathrm{rg} \sin \phi\) \(\frac{\mathrm{v}^{2}}{\mathrm{rg}}=2 \cos \theta-2 \sin \phi \ldots \ldots(\mathrm{ii})\)…

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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