My tiny 7th grade self wanted to figure out the science behind trick shots in pool, so I could improve my game (because as a 12-year-old I was obviously very involved in the professional pool circuit). Conveniently, my dad worked for a robotics company (Adept Technology) and brought me home an Adept Cobra i800 4-axis robot.
When you hit the cue ball at an angle with respect to the object ball, you create cut-induced throw on the object ball, causing the object ball to travel tangent to the impact line. When you hit the cue ball with a clockwise or counterclockwise spin (accomplished by hitting the cue ball off center), you create spin-induced throw on the object ball, and the sliding friction propels the object ball to throw left or right (for clockwise or counterclockwise spin respectively). In my experiment, I attached a cue ball to the robot so I could hit an object ball with cut, spin, and a combination of both. Using simple trigonometry, I predicted the object ball’s trajectory in each shot.
The results? I’m now marginally better at pool. And there are permanent scuffs in my parents’ pool table from robotic abuse.
The effects of cut-induced throw, spin-induced throw, and a combination of both (total-induced throw).
Setup. I used a wooden fixture to properly position the cue and object balls. In half of the tests, I placed a rubber band around the cue ball to create more friction.
Small child (me) collecting data on final position of object ball.
A prediction of the effect of 7° cut angle.
Simple code driving the robot to strike the object ball with the cue ball.