This is the fourth of a 4-part series on billiard physics outlining the 4 major contact points: cue stick →  cue ball, ball → table, ball→ ball, ball → rail.

Spherical balls glancing off each other on a flat surface may sound simple, but there are many physical forces that apply on the pool table. Knowing how and why the balls react the way they do will greatly improve your learning curve by helping you understand why you missed a particular shot or position.

You need to know the effects when a ball contacts a rail for most cue ball positioning, bank shots, and kick shots.

• Loss of Speed
  When a ball contacts a rail, it will lose a portion of its speed.
  Tip: Every table and rail plays different; pay attention to how much speed is lost when a ball hits a rail.
• Speed & Rebound Angle
  At low to medium speeds, a ball will rebound off of a rail at near the same angle that it went into the rail. At higher speeds, the rebound angle is shortened.
  Tip: Pay attention to the variance in tables and rails for how much angle you can shorten, based on the speed of the ball.
• Left/Right Spin & Rebound Angle
  If the cue ball has left or right spin when it contacts a rail, the rebound angle will change in the direction of the spin.
  Tip: Use left and right spin to play position off of a rail.
• Top/Bottom Spin & Rebound Angle
  If the cue ball has top or bottom spin when it contacts a rail, the cue ball will curve (bend) similar to the effect of top and bottom when contacting an object ball.
  Tip: Be aware of the bending effect of top and bottom spin when kicking at a ball.
• Spin & Rebound Speed
  If the cue ball has matching spin to the angle of approach to the rail, this is referred to as “running English” and will increase the speed of the cue ball. If the cue ball has the opposite spin to the angle of approach, this is referred to as “hold-up English” and will decrease the speed of the cue ball. This effect is increased by more spin and less angle.
  Tip: Use running and hold-up English to play position off of a rail.
• Induced Spin
  When a ball contacts a rail at an angle, a small amount of sidespin will be imparted to the ball in the direction of the angle.
  Tip: Be aware of induced spin when using multiple rails for positioning the cue ball.
• Rail Compression
  When a ball contacts a rail, the rail will compress allowing the ball to go into the rail. This effect is increased with higher speeds and less angle.
  Tip: Recognize shots that can only be made using a rail compression. For example, an object ball that is barely blocked by the point of a side pocket.