Question

A ball of mass $ 0.6\,kg $ attached to a light inextensible string rotates in a vertical circle of radius $ 0.75\,m $ such that it has a speed of $ 5\,ms^{-1} $ when the string is horizontal. Tension in string when it is horizontal on other side is $ (g=10\,ms^{-2}) $

• A. 30N
• B. 26N
• C. 20N
• D. 6N

Answer 1

Answer: (C)

Tension in the string when it makes angle $\theta$ with the vertical,
$T=\frac{m v^{2}}{r}+m g \cos \theta$
When the string is horizontal, $\theta=90^{\circ}$
Here, $m=0.6\, kg , r=0.75\, m , v=5\, m / s$
Hence, $ T=\frac{m v^{2}}{r}+m g \times 0=\frac{m v^{2}}{r} $
$ \therefore T=\frac{0.6 \times(5)^{2}}{0.75}=20\, N$