Mechanics 1 - pulley question.

Hi, i don't really get question (e), is there anyone explain question (e)?

Here is the question.

6. Two particles P and Q have mass 0.5 kg and m kg respectively, where m < 0.5. The particles are connected by a light inextensible string which passes over a smooth, fixed pulley. Initially P is 3.15m above horizontal ground. The particles are released from rest with the string taut and the hanging parts of the string vertical. After P has been descending for 1.5 s, it strikes the ground. Particle P reaches the ground before Q has reached the pulley.

acceleration is 2.8 ms^-2

Tension is 3.5 N

and m = 5/18

(e)

When p strikes the ground, P does not rebound and the string becomes slack. Particle Q then moves freely under gravity, without reaching the pulley, until the string becomes taut again.

hence, find the time between the instant when P strikes the ground and the instant when the string becomes taut again.

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Mark scheme says

v = u + at => 4.2 = -4.2 + 9.8t

therefore, t = 6/7

but i dont really get this.. could anyone explain why v is -4.2 too??

Thank you.