Pool sharks generally have a plan for pocketing balls. The simplest and easiest way to pocket the balls is usually the most successful, but more knowledge allows for more options.
Pool is learned by experience and observation, but the laws of physics can also be helpful. Since the balls are the same size and mass, a direct hit from a moving ball on one at rest merely transfers the speed to the moving ball to the one at rest, if ball spin does not also determine the result. As for spin, the friction of the table causes the moving ball to roll. The roll itself depends on the amount of force applies to the cue ball and the distance between the cue ball and the object ball, as more distance equals more friction. If the cue ball is struck hard in the center with the cue stick, it slides a longer distance for no roll to occur, thus stopping after a straight center hit on the object ball. If the ball is struck easier, then roll takes effect at a shorter distance.
If the transfer of speed is not in a straight line, and there is no spin on the cue ball, then the cue ball and object ball move at angles 90 degrees apart. With a 45 degree cut, the cue ball and object ball move at the same speed. With less than a 45 degree cut, the object ball then moves faster than the cue ball. With more than a 45 degree cut, the cue ball then moves faster than the object ball. A direct hit stops the cue ball. A 90 degree cut does not move the object ball. A near 90 degree cut moves the object ball farther if the cue ball is struck harder.
How hard the cue ball is hit determines where the cue ball ends up, as for an easier shot on the next object ball. Speed control is thus essential, but different ways to spin the cue ball can also be an advantage. Besides the friction of the table, a smooth follow through of the cue stick for more impulse and restoring force of the cue stick tip results in more spin of the cue ball. A backward spin by striking the cue ball below center draws it back after it hits the object ball. Backward spin is more difficult to obtain due to the friction of the table, and it is even more difficult with longer distance between the cue ball and the object ball. With forward spin, the 90 degree angle is decreased. With backward spin, it is increased.
English in pool refers to either clockwise or counterclockwise spin given to the cue ball by striking it either left of right of center. Striking the cue ball left of center initially pushes it to the right of the stroking direction, but the clockwise spin of the cue ball and the friction of the table causes the cue ball to curve to the left. The amount of curve is according to the ratio of spin and forward speed of the ball. A hard struck cue ball keeps it more to the right, whereas an easier strike with smooth follow through farther from the center of the cue ball curves it with more spin more to the left. The ratio of spin can also be determined by the vertical angle of the cue stick and where it strikes the cue ball. An acute downward stroke above center of the cue ball curves it back at shorter distance. A stroke below center of the cue ball curves it with more backward spin less until it curves more sharply back at a longer distance from where it was struck. Generally, a ball obstructing a straight path is a guide ball to determine how the cue ball should be struck.
Generally, a slower forward speed allows a ball to curve more. Even an object ball curves a little from the spin of the cue ball, as having a lot of English with slower forward speed. With tables having a lot of friction, as because of less stretch of the cloth, an angle hit of the cue ball on the object ball provides a little table roll for increasing the angle of the object ball more towards the pocket.
A spinning ball also changes the direction of the ball it collides with, which in pool language is called throw. It also transfer spin to the other ball. Thus right English given to the cue ball spins it clockwise, which spins the object ball counterclockwise. A direct hit of the object ball on another ball then caroms the object ball to the right, as faster and more than normal. Ball spin also either increases or decreases the angle and speed of the ball after it hits a rail of the table.
About Author / Additional Info:
Rice is nice but Bob Ticer is nicer. I began shooting pool my last of four years in the Air Force. In the air base library was an old book explaining the pool shooting of an retired 20 year undefeated world champion of snooker. His name was Joe Davis.