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# Dynamics using moments watch

1. two particles P and Q of masses 3kg and 2kg respectively are at rest on a smooth horizontal surface. They are connected by a light inelastic string which is initially slack. P is projected away from Q with a speed of 10 m/s . Find the common speed of the particles after the string becomes taut and the impulse in the string when it jerks tight.

I have managed to find the speed of the particle as 6 m/s but I don't understand how to find the impulse in the string. I know it is twelve, but my method of working it out is insane. Please can someone kindly show me step by step how to work it out. Please also explain to me the correct directions the force must be etc..

Thanks

Additionally can someone show steps for this question :
Particle A of mass 1 kg is at rest 0.2m from the edge pf a smoooth horizontal 0.8m high table. It is connected by a light inextensible string of length 0.7m to a particle B of mass 0.5kg. Particle B is initially at rest at the edge of the table closest to A but then falls off. Asuming B's initial horizontal belocity to be zero, find the speed with which A begins to move.
2. Impulse = change in momentum

Particle P:
masss = 3 kg
u = 10 m/s
v = 6 m/s
Δv = u - v (change in velocity) always positive since it's just a change
Δv = 10 - 6 = 4
I = mΔv = 3*4 = 12
I = 12 Ns
=======

Particle Q:
masss = 2 kg
u = 0 m/s
v = 6 m/s
Δv = v - u (change in velocity) always positive since it's just a change
Δv = 6 - 0 = 6
I = mΔv = 2*6 = 12
I = 12 Ns
=======
3. 3*10 + 2*0 = 3*V + 2*V

30 = 5V
V = 6m/s

I = mv - mu = 3*6 - 3*10 = 18 - 30 = -12
I = 12
4. Is this a past paper question??? If so what paper is it from and also can you provide the answer to the second question....?
Additionally can someone show steps for this question :
Particle A of mass 1 kg is at rest 0.2m from the edge pf a smoooth horizontal 0.8m high table. It is connected by a light inextensible string of length 0.7m to a particle B of mass 0.5kg. Particle B is initially at rest at the edge of the table closest to A but then falls off. Asuming B's initial horizontal belocity to be zero, find the speed with which A begins to move.
B will fall under gravity for 0.5 m (0.7 - 0.2).
Find its speed at this point, which is when the string becomes taught, then do as for the previous question. Find the common speed.
6. (Original post by Fermat)
B will fall under gravity for 0.5 m (0.7 - 0.2).
Find its speed at this point, which is when the string becomes taught, then do as for the previous question. Find the common speed.
thank you fermat and revelation thats great thanks

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