...
The cue ball is moving at 54.0 m/s when it impacts the 6 5-ball, which is at rest prior to the collision. The cue ball exits the (perfectly elastic) collision at an angle of 30 degrees from its original direction of motion. What are the final speeds of each ball?
System: Cue ball and 5-ball as point particles. External forces are assumed negligible compared to the collision forces.
Models: Momentum and Impulse plus Constant Mechanical Energy.
Approach: We begin with a picture. Since the initial direction of motion of the cue ball is not specified, we arbitrarily assign it to move along the x-axis prior to the collision. We also arbitrarily assign it positive x- and y-velocity components after the collision.