PRACTICE / Circular force balance

What speed keeps the string taut at the top?

A small mass attached to a light string of length 2 m moves in a vertical circle. Take g = 10 m/s². The string can pull but cannot push.

What is the minimum speed at the bottom for a complete circle with the string just taut at the top?

1. A$10\ \mathrm{m\,s^{-1}}$
2. B$\sqrt{20}\ \mathrm{m\,s^{-1}}$
3. C$\sqrt{80}\ \mathrm{m\,s^{-1}}$
4. D$20\ \mathrm{m\,s^{-1}}$

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