Spin on a pool ball is rotational motion imparted to the ball when it is struck by the cue stick off-center. This spin significantly affects the ball's trajectory, interaction with the table cloth, and behavior after hitting other balls or cushions.
When you hit the cue ball precisely in its center, it travels forward without significant rotation relative to the table surface (initially skidding, then rolling). However, hitting the ball away from its vertical or horizontal center point introduces different types of spin, often referred to as "English" in pool.
Types of Spin and Their Effects
The type of spin depends on where the cue tip strikes the cue ball:
- Topspin (Follow): Hitting the top of the ball causes it to rotate forward. This makes the cue ball roll forward faster after hitting an object ball.
- Bottom Spin (Draw): Hitting the bottom of the ball causes it to rotate backward. This makes the cue ball reverse direction or stop after hitting an object ball.
- Side Spin (Left or Right English): Hitting the left or right side of the ball imparts rotation around a vertical axis. This is where things get interesting regarding the ball's initial path.
The "Pushing Off" Effect with Side Spin
Side spin not only affects how the ball reacts after impact but also influences the initial path of the cue ball, especially on longer or harder shots.
According to one perspective, Hitting with left spin pushes the cue ball slightly off to the right and right spin pushes the cue ball off slightly to the left. This effect, often called "squirt" or "deflection," happens because the off-center contact causes the cue stick itself to deflect slightly in the opposite direction, subtly altering the cue ball's path as it leaves the tip.
- Conditions for Pronounced Effect: This pushing off effect is more pronounced the harder the cue ball is struck and the further away from the vertical center the shot is struck.
- Equipment Variation: It's important to note that this effect varies greatly based on the type of cue that you play with, as different cue constructions (especially the shaft) exhibit different deflection characteristics.
Why Does Spin Alter Trajectory and Roll?
The primary reason spin affects the ball's motion is friction with the table cloth.
- Side Spin: As a side-spinning ball slides across the cloth, the friction force acts against the direction of the spin. This force causes the ball to curve slightly over distance after the initial squirt effect from the cue. A ball hit with left spin will initially squirt right but then curve back left as the spin takes hold and interacts with the felt.
- Topspin/Bottom Spin: These spins affect how quickly the ball transitions from skidding to rolling and how it interacts with the cloth and other balls after collisions, influencing follow-through or draw effects.
Understanding and controlling spin is a fundamental skill in pool, allowing players to position the cue ball for the next shot, navigate obstacles, and execute complex shots.
