Homework Help: Two pendulums banging into each other problem.

1. Aug 20, 2013

tjhero123

1. The problem statement, all variables and given/known data

A system contains two metal balls hanging from strings of length !.0 m. Pendulum A has a mass of 2.0 kg and B has a period of 2.0s. They are released from the same height at the same time and collide right in the middle of their arcs. If pendulum A comes to a complete stop while B moves backwords with half of its original speed, what is the mass of pendulum B?

2. Relevant equations

Conservation of momentum.

3. The attempt at a solution

My first thought was the conservation of momentum. 2Va+MbVb=-(1/2)MbVb. Of course theres 3 variables so i couldnt solve it but then i thought that since the balls were colliding in the middle of their arcs that maybe they're velocities were the same. So Va=Vb. From that logic I get that the mass of B is -4/3 which obviously makes no sense..... Im not sure what im doing wrong. Im gratful for any help. Thanks in advance

2. Aug 20, 2013

ehild

The momentum is a vector. The balls move in opposite directions just before they collide. Va is not equal to Vb.

ehild

3. Aug 21, 2013

tannerbk

Use the equation for the period of a pendulum to help you find Vb.

4. Aug 21, 2013

haruspex

The time taken is irrelevant. You cannot deduce any actual velocity since you do not know the original height. ehild's hint is the way forward.