In this question use $g=9.8$ m s${}^{-2}$.

Two particles $P$, of mass 4 kg, and $Q$, of mass 5 kg, are connected by a light inextensible string passing over a smooth pulley fixed at the top of a smooth plane inclined at an angle $\theta$ to the horizontal, where $\cos \theta =0.8$.

$P$ lies at rest on the plane with the string parallel to a line of greatest slope, and $Q$ hangs freely 2 m above the ground with the string taut.

The system is released from rest. Assume that $Q$ reaches the ground before $P$ reaches the pulley.