Question

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

• A. 15 cm
• B. 4 cm
• C. 9 cm
• D. None of these

Answer 1

Answer: (C)

Velocity of bob at C should be $\sqrt{5gR}$

$\sqrt{5g(R-L)}=\sqrt{2gR}$
$5(R-L)=2R$
$3R=5L$
$L=\frac{3}{5}R=\frac{3}{5}\times 15=9cm$