Question

A stone is tied at one end of a $5m$ long string and whirled in a vertical circle. The minimum speed required to just cross the topmost position is:

• A. $5{ms}^{-1}$
• B. $7{ms}^{-1}$
• C. $57{ms}^{-1}$
• D. $75{ms}^{-1}$

Answer 1

The correct option is B $7{ms}^{-1}$

Tension at the highest point of the circle is:

$\frac{m{v}^2}{r}=mg+T$

For minimum speed at the top tension in the string will be zero.

In that case $v=$$\sqrt{rg}$, where $g=9.8m{s}^{-2}$

$v=\sqrt{49}$

So, $v=7m{s}^{-1}$.