Question

A ball weighing $1.0kg$ is tied to a string $15$ cm long. Initially the ball is held in position such that the string is horizontal. The ball is now released. A nail $N$ is situated vertically below the support at a distance $L$.What is the minimum value of $L$ (in cm) such that the string will be wound round the nail?

Answer 1

Answer: 9

Velocity of bob at $C$should be$\sqrt{5gR}$
$\sqrt{5g\left(R-L\right)}=\sqrt{2gR}$
$5\left(R-L\right)=2R$
$3R=5L$
$L=\frac{3}{5}R=\frac{3}{5}\times 15=9cm$