Physics textbooks often use billiards in examples to explain concepts in mechanics - but they teach you very little about how physics can be used in billiards.
Billiards, pool, and cue sports in general all revolve around hitting hard little balls with slender sticks. These are games of finesse, concentration, and strategy. A knowledge of the physics underlying the game play is imminently useful.
There are four important physical priniciples at work in basic game play. These are:
- velocity
- acceleration
- momentum
- impulse
Velocity is simply the movement of the balls across the table. The velocity is constantly changing (acceleration), because of friction between the balls and the felt. Momentum is mass times velocity, and is a conserved quantity. When one ball hits another, the first will impart velocity to the second through momentum. When the cue ball hits another ball, the other ball will move with similar velocity, but the cue ball will slow, if not halt altogether. The total momentum of the system, before, during, and after the hit remains the same.
Since the balls are manufactured to be the same mass, the amount of velocity transferred between balls is reasonably predictable. If the cue ball was more massive than the other balls, it would impart much more velocity to the object ball, and if it was less massive, it would be relatively ineffective at transferring velocity.
Impulse is the change in momentum of an object over time. It describes the hit and transference of velocity from cue ball to object ball, or cue stick to cue ball.
Friction
One important aspect of the game is the felt on the table. A billiard table is made to be as flat as possible, with few surface imperfections. This insures that game play is predictable and consistent between tables. The felt provides traction over this flat surface and helps to further remedy imperfections. The felt also slows down the movement of the balls considerably. Friction is a force, and force is measured as mass times acceleration. As the ball rolls across the table, the fibers in the felt gently push on the surface of the ball. This may seem rudimentary, but it is an important part of understanding spin in billiards.
Billiard Ball Spin
Spin is another word for angular momentum - mass times rotational velocity. Spin is imparted to a ball when it is hit off center, for example, above or below it's horizontal or vertical planes (imagine the ball cut in two down the middle, the place that it is cut is the vertical plane). There are three types of billiard spin:
- follow
- draw
- sidespin or english
English is perhaps the most well-known type of spin for amateur players, simply because it is a memorable name. English is caused by hitting the ball somewhere on either side of the vertical plane. Follow is caused by hitting the ball above the horizontal plane, and draw is caused by hitting below the horizontal plane. Really it's all the same thing, just different directions.
Since spin is a form of momentum, it is conserved (unless significantly acted upon by friction), and can be imparted from one ball to another. A ball with follow will impart some draw and vice versa. Follow is also a result of friction from the felt. When a ball is hit dead center, it will slide on the felt, then will start rolling forward as the felt pushes back on the underside of the ball. A ball with draw will eventually lose it to the follow from the felt.
