The Facts About the Spinning Cue Ball

By Steve Jennings

The physics of a spinning cue ball and the different results they produce may be one of the most misunderstood subjects in pool. Today, I want to try to clear up some questions and help understand how different kinds of spin produce different results.

There are two basic types of spin that can be applied to the cue ball, and they are quite different. Top and bottom spin applied on the vertical centerline will determine the path the cue ball takes after it contacts the object ball. Left or right side spin, often called English, changes the rebound angle when the cue ball contacts the cushion. Both are important to understand to play position for another shot.

Most of us know that on a straight-in shot, applying topspin will allow the cue ball to follow the object ball after contact, while using bottom spin can cause the cue ball to travel back with draw. If there is neither forward nor backward spin upon contact, the result is a stop shot, where the cue ball remains at the point of contact.

All that changes when there is any cut angle on the shot. After contact on any cut shot, the cue and object balls will separate on a 90-degree angle from the line of centers of the two balls. This path is often called the tangent line. If the cue ball has forward spin, it will then roll forward of the tangent line, while backspin will cause it to curve back off the tangent line. Only when there is no forward or back spin at contact will the cue ball continue along the tangent line. Side spin actually has very little impact on the path of the cue ball after it separates from the object ball. It can radically change the rebound angle the cue ball takes when it hits the cushion. For clarity, we refer to either inside or outside English. If your shot is cutting a ball to the left, then left spin would be inside, and right spin would be outside. For a cut shot to the right, the opposite would be true. Just remember that inside is the side closer to the pocket you’re shooting toward.

Without any sidespin, the cue ball will rebound off the rail at approximately the same angle as it approached it. So if the cue ball comes into the rail at a 30 degree angle, it will rebound off the rail at about 30 degrees. Note that the speed of the ball may alter the rebound angle. But if the cue ball hits the cushion at a 30 degree angle with outside spin, it will come off at an angle greater than 30 degrees. Inside spin will shorten the rebound angle to less than 30 degrees.

Adding more sidespin by striking the cue ball farther away from the vertical centerline will increase the amount of change to the rebound angle.

It’s important to know that applying sidespin to the cue ball will cause it to move slightly off the aim line. Left spin will push the cue ball slightly to the right, and right spin will push it to the left. How much will vary based on how much spin is applied.

The weight and composition of your cue, along with speed, will impact the amount of deflection you will see. Most instructors will encourage players to focus primarily on using top and bottom along with speed for position play whenever possible. Sidespin will make every shot a little bit more difficult. To paraphrase Jerry Briesath, the cue ball doesn’t care if sidespin is applied intentionally or by accident, it will still deviate from the original aim line.

I hope this helps clear up any misunderstanding about how applying spin to the cue ball will affect the shot. If you would like more on this topic, feel free to contact me or another qualified professional instructor.

Until next time.

Steve Jennings PBIA/ACS Level 4 Master Instructor