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A heavy, big sphere is hung from a string of length $\ell$. The sphere moves in a horizontal circular path making an angle $\theta$ with the vertical. Its time period is:
- $T=2\pi\sqrt{\frac{\ell}{g}}$
- $T=2\pi\sqrt{\frac{\ell\sin\theta}{g}}$
- $T=2\pi\sqrt{\frac{\ell\cos\theta}{g}}$
- $T=2\pi\sqrt{\frac{g}{\ell\cos\theta}}$
Correct answer: $T=2\pi\sqrt{\frac{\ell\cos\theta}{g}}$
Solution
This is a conical pendulum. From vertical balance, $T\cos\theta=mg$, and from horizontal centripetal force, $T\sin\theta=m\omega^2 r$ with $r=\ell\sin\theta$. Combining these gives $\omega^2=g/(\ell\cos\theta)$, so the time period is $2\pi\sqrt{\ell\cos\theta/g}$.
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