Q 11-05-040NEETJEE MainEasy
A stone tied to a string of length $0.4$ m is whirled in a vertical circle. The minimum speed it must have at the lowest point to just complete the circle is ($g = 10\ \text{m/s}^2$)
Answer: (C) $\sqrt{20}$ m/s
$$v_{min} = \sqrt{5gr} = \sqrt{5 \times 10 \times 0.4} = \sqrt{20} \approx 4.5\ \text{m/s}$$
(At the top it then has the minimum speed $\sqrt{gr} = 2$ m/s, with zero tension.)
Solution by Sreeraj P, M.Sc Physics