A bob is whirled in a horizontal plane by means of a string with an initial speed of $\omega$ rpm. The tension in the string is $T$. If speed becomes $2 \omega$ while keeping the same radius, the tension in the string becomes:

• Correct answer
  A

  4T
• B

  T/4
• C

  $\sqrt{2} T$
• D

  T

Solution

$T=m \ell \omega^2$
$\begin{aligned} & T^{\prime}=m \ell(2 \omega)^2 \\ & T^{\prime}=4 T\end{aligned}$

Hence, the correct answer is option: 'A'.