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# M1- Dynamics watch

1. Two particles A and B have mass 0.4 kg and 0.3 kg respectively. The particles are attached to
the ends of a light inextensible string. The string passes over a small smooth pulley which is
fixed above a horizontal floor. Both particles are held, with the string taut, at a height of 1m
above the floor, as shown in the diagram above. The particles are released from rest and in the
subsequent motion B does not reach the pulley.

When the particles have been moving for 0.5 s, the string breaks.
(c) Find the further time that elapses until B hits the floor

If a particle falls down then the velocity is negative but what about displacement
I am stuck at this question, if anyone could help i would be forever grateful !
2. (Original post by gcsehelpxoxo)
Two particles A and B have mass 0.4 kg and 0.3 kg respectively. The particles are attached to
the ends of a light inextensible string. The string passes over a small smooth pulley which is
fixed above a horizontal floor. Both particles are held, with the string taut, at a height of 1m
above the floor, as shown in the diagram above. The particles are released from rest and in the
subsequent motion B does not reach the pulley.

When the particles have been moving for 0.5 s, the string breaks.
(c) Find the further time that elapses until B hits the floor

If a particle falls down then the velocity is negative but what about displacement
I am stuck at this question, if anyone could help i would be forever grateful !
At the instant the string breaks it will have an upwards velocity u and will be at height h above the ground. You have to work out u and h from the motion of the particle before the string breaks. (And don't forget that the particle starts 1m above the ground so the 1m has to be included in h).

When the string breaks the particle become a projectile. You can think of it as being like one of those problems where a projectile is thrown into the air (with upwards velocity u) and you have to work out the time taken until the particle reaches a given height, h below the starting point in this case.

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Updated: February 18, 2018
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