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. When released from the initial horizontal position, its instantaneous angular acceleration will be\\
(1) $\frac { g } { 2l }$\\
(2) $\frac { 7g } { 3l }$\\
(3) $\frac { g } { 3l }$\\
(4) $\frac { g } { 13l }$