You are up in the middle of the night watching the World Cup of Trick Shots (the

Question

You are up in the middle of the night watching the World Cup of Trick Shots (there is nothing else on at 3:30 am this particular evening). The contestant hits the cue ball such that it runs into a few other balls and eventually bounces back around on a collision course with a second, striped ball. All other balls are out of the way of this particular collision. Directly before the collision, the cue ball is traveling in the +y-direction at a speed of 4.2 m/s and a striped ball is traveling in the +x-direction at a speed of 2.6 m/s. They collide elastically at the origin of the xy-coordinate system. After the collision, the striped ball moves in the +ydirection. Assume all balls on the table are of equal mass. a. What is the final direction of the cue ball? b. What are the speeds of the two balls directly after the collision?

Explanation / Answer

a) Cue ball was moving in +Y direction at 4.2 m/s before collision and striped ball was moving in +X-direction at 2.6 m/s before collision

after collision , striped ball moves in +Y direction

by symmetry , striped ball moves at 4.2 m/s in +Y-direction to keep the momentum before and after same

by symmetry , cue ball moves at 2.6 m/s in +X-direction to keep the momentum before and after same

b) cue ball ---   2.6 m/s

striped ball -----4.2 m/s

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