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So this is less about about the physics problem and more about the concepts behind it. I don't need help with any solution but I am framing my questions around the example given.

So there are two steel balls with equal mass that are hanging from a point by a string. Both strings are the same length. One ball is lifted so that one string makes an angle to the other string. The ball is let go and swings downward along a circular path and hits the other steel ball elastically.

So these are questions/confusions.

When speaking of the momentum of the masses, assuming that the system is made up of both steel balls combined, the momentum is conserved. Kinetic energy is also conserved. This means that the momentum of ball one (the ball that was lifted) is equal to the momentums of both steel balls after the collision.

The kinetic energy of ball one right before the collision is equal to the kinetic energy of both balls after the collision.

When looking up solutions for this homework problem, I read that the velocity after the collision of ball one (the ball that was picked up) is zero. So that all momentum of ball one before the collision is transfered to ball two after the collision.

But the solution doesn't explain, and I can't figure out through my math, how they arrive at this conclusion.

Couldn't it also be that ball one hits ball two and both have velocities? I would think that in doing this experiment, ball one wouldn't just "stop" and ball two starting moving....

Thanks!

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# I Momentum, Energy, and Elastic Collisions

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