Question

A rigid massless rod of length $3l$ has two masses attached at each end as shown in the figure. The rod is pivoted at point $P$ on the horizontal axis (see figure). When released from initial horizontal position, its instantaneous angular acceleration will be :

• A. $\frac{g}{2l}$
• B. $\frac{7g}{3l}$
• C. $\frac{g}{13l}$
• D. $\frac{g}{3l}$

Answer 1

Answer: (C)

$\frac{g}{13l}$

Answer 2

Applying torque equation about point P.
$2M_{0} \left(2l\right)-5 M_{0} gl = I\alpha$
$I = 2M_{0}\left(2l\right)^{2} + 5M_{0} l^{2} = 13 M_{0} l^{2}d$
$\therefore \alpha = - \frac{M_{0}g \ell}{13M_{0} \ell^{2}} \Rightarrow \alpha = - \frac{g}{13\ell}$
$\therefore \alpha = \frac{g}{13\ell}$ anticlockwise