Newton's laws of motion — practice question

Particles $P$ and $Q$ together have a total mass of $1\text{ kg}$. They are connected by a light inextensible string that runs over a smooth fixed pulley. $P$ is kept at rest while $Q$ hangs freely, and both vertical sections of the string are straight. At the start, both particles are at a height of $h\text{ m}$ above the floor (see Fig. 1). $P$ is released from rest and the particles begin to move with the string taut. Fig. 2 gives the velocity-time graphs for the motion of $P$ and for the motion of $Q$, with vertically upwards taken as the positive direction for velocity.