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
Concept. Minimum speed at the bottom to complete a vertical circle: $\sqrt{5gr}$.
Step 1. Substitute. $\sqrt{5 \times 10 \times 0.4} = \sqrt{20}$ m/s.
Answer. $\sqrt{20}$ m/s (Option C).
Solution by Sreeraj P, M.Sc Physics