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A cane filled with water is revolved in a vertical circle of radius $4\,\text{m}$, and the water just does not fall down. The time period of revolution will be

1. 1 s
2. 10 s
3. 8 s
4. 4 s

Correct answer: 4 s

Solution

If the water just does not fall at the top, the required centripetal acceleration equals $g$, so $v^2/r=g$. Thus $v=\sqrt{gr}=\sqrt{10\times4}=\sqrt{40}$. The time period is $T=2\pi r/v\approx 2\pi\cdot4/\sqrt{40}\approx 4\,\text{s}$.

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