This is the second 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.

Every ball contacting the table is in one of 3 states: not moving, rolling, or sliding.

• Normal Roll
  A ball rolling on the table without any sliding is called normal roll. A ball that has normal roll will continue to go in a straight line until it contacts another ball, rail, or slows down enough to stop.
  Tip: Use normal roll for the most consistent line of aim.
  Tip: Hit the cue ball about one tip above center to immediately impart normal roll.
• Loss of Speed & Spin
  Over time, a ball moving on the table will gradually lose any speed and spin due to the friction between the ball and the table. Most spin will wear off quickly, but it takes a long distance for left or right spin to be completely lost.
  Tip: Be aware of the amount of spin you are putting on the cue ball for the distance you want the cue ball to travel.
• Top & Bottom Sliding
  Top sliding is when the cue ball over-rotates on the table, causing it to speed up. Topspin will cause the cue ball to go forward after colliding with an object ball, after traveling down the tangent line. Bottom sliding is when the cue ball under-rotates, causing it to slow down. Bottom spin will cause the cue ball to travel backward after colliding with an object ball, after traveling down the tangent line.
  Tip: Use top or bottom spin to change the direction and speed of the cue ball after contact with an object ball.
  
• Curve (Swerve)
  Hitting the cue ball with left or right spin will cause it to curve (swerve) in that direction on the table. Elevating the cue stick will increase the effect of the curve. Hitting the cue ball with more speed will cause the curve to happen at a greater distance.
  Note: Elevating the cue stick above 45 degrees to cause a significant curve is referred to as a massé shot. 
  Tip: Try to keep your cue stick as level as possible to reduce the amount of curve.
• Jump Shot
  Hitting the cue ball near the center with some elevation will cause the cue ball to rebound off of the table into the air, which is known as a jump shot. The more elevation and speed will increase the effect of the jump.
  Tip: Use a lighter cue stick to jump the cue ball easier.