Newton's laws of motion — practice question

Particles $P$ and $Q$, with masses $0.6\,\text{kg}$ and $0.4\,\text{kg}$ respectively, are joined by a light inextensible string. This string runs over a small smooth light pulley that is fixed at the end of a smooth horizontal table. At first, $P$ is held stationary on the table while $Q$ hangs vertically (see diagram). $P$ is then released.