Ball 1, with a mass of 20kg is moving to the right at 20 m/s. at what velocity should ball 2, with a mass of 40kg, move so that they both come to a standstill upon collision?

This is a conservation of momentum question. Initial momentum of the system is the momentum of ball 1 plus the momentum of ball 2. The final momentum of the system should be 0 since the balls stand still after the collision.

mv + mv = 0

mv = -mv

(20)(20) = -(40)Vi

400 = -40Vi

Vi = -10

So ball 2 should travel at 10m/s to the left

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