Two billiard balls have the same mass. The 2-ball is at rest, and the 8-ball moves horizontally towards it at 3 m/s.
8 ---> 2After the balls collide, which of the following results is impossible? Explain.
Case A: 8 2 ---> Case B: 2 --> 8 \ \ / / 2 Case C: 8 \ \
Answer: The key is to ask if total momentum is conserved in each case. The original total momentum is entirely directed to the right -- that means that the final total momentum must be completely directed to the right. In case A, final momentum is directed to the right. Possible. In case B, some of the final momentum is directed up. Impossible. In case C, the final momentum may still be directed to the right, if the vertical momentum of the 8-ball cancels that of the 2-ball. Possible.
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