# Thread: Mechanics 1 - Pulley Question, Why does the rope become slack when particle moving?

1. ## Mechanics 1 - Pulley Question, Why does the rope become slack when particle moving?

i've managed to do parts a,b,c,d ,e but not f.

Figure 4 shows two particles P and Q, of mass 3 kg and 2 kg respectively, connected by a light inextensible string. Initially P is held at rest on a fixed smooth plane inclined at 30° to the horizontal. The string passes over a small smooth light pulley A fixed at the top of the plane. The part of the string from P to A is parallel to a line of greatest slope of the plane. The particle Q hangs freely below A. The system is released from rest with the string taut.
Acceleration = 0.98 ms-2
T= 18N

On release, Q is at a height of 0.8 m above the ground. When Q reaches the ground, it isbrought to rest immediately by the impact with the ground and does not rebound. Theinitial distance of P from A is such that in the subsequent motion P does not reach A.
Find(e) the speed of Q as it reaches the ground,(2)
v= 1.25ms-1

(f) the time between the instant when Q reaches the ground and the instant when thestring becomes taut again.

I get how you tackle part f but i dont get why the tension would be lost in P? could someone please explain...

2. ## Re: Mechanics 1 - Pulley Question, Why does the rope become slack when particle movi

P is moving up the incline when Q hits the ground and has sufficient momentum to continue moving up the incline causing slack in the string.

Q hits the ground at v = 1.25 m/s $\implies$ P's velocity is also v = 1.25 m/s up the incline at Q's impact time ... P will move an additional $t = \dfrac{v}{g\sin(30)} \, sec$ up the incline ... it will take the same amount of time for P to return to its position when Q hit the ground.