When the cue ball strikes an object ball at an angle, they separate at exactly 90 degrees (for a perfectly elastic, frictionless collision). In reality, due to friction during contact, the angle is slightly less than 90° (typically 85–88°).
Law:
[
\vecPtotal, initial = \vecPcue, final + \vecP_object, final
] the physics of pocket billiards pdf
Striking the cue ball off-center produces combinations of linear and rotational velocity. When the cue ball strikes an object ball
| Strike Point | Effect | Physics Explanation | |--------------|--------|----------------------| | Center (0) | Follow (roll) | No initial spin; friction quickly creates forward roll. | | Above center | Top spin / follow | Forward angular momentum causes ball to accelerate after contact. | | Below center | Back spin / draw | Reverse spin persists; upon object ball impact, friction reverses direction of cue ball. | | Left/right | English (sidespin) | Angular momentum around vertical axis changes rebound angle off rails. | | Strike Point | Effect | Physics Explanation
No analysis of billiards physics is complete without discussing rotational dynamics. Most searching for a "physics of pocket billiards pdf" want to demystify spin, or "English."