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

How do I solve this?

3 answers

conserve momentum. After the collision, the total momentum is zero. So, it must also come out to zero while the balls were moving:

20*20 + 40v = 0

Note the sign of the velocity for B. What does that indicate?
Wait so I'm trying to solve for the V?
well, yeah - that was the question, wasn't it?