Realism was always the main aim for both Snooker147 and Poolster, and the billiard engine lets us approach this ideal. Even although both these games (perhaps more accurately, simulations) are 2D, the realism of the ball behaviour has been highly praised by many customers. This realism is brought about by 2 main things: collisions and ball spin.
Ball Spin
The property which distinguishes the motion of a billiard-ball from, say, a puck on ice, is spin. At any instant, the ball has some spin and some velocity, and because of friction with the table cloth, these interact and energy is exchanged between them.
For example, if a stationary ball is struck in its centre with a cue, the ball will initially move with a given velocity and it will have zero spin. However, as the ball travels forwards, friction between the ball and the table cloth will cause the ball to roll, rather than slide (as would happen to a puck on ice). By definition, if the ball is rolling it is spinning, therefore the ball gains spin as it travels. Eventually, the amount of spin will equal the speed of ball travel; this is what we mean when we say the ball is rolling i.e., it has no excess spin.
However, if our moving ball collides with another ball head-on, it will stop, but the spin will remain; although now the spin will be excess spin because it is larger than the ball's velocity (which is now zero). This spin, through interaction with the table cloth, will cause the ball to begin to accelerate in a forward direction.
As the ball moves (its displacement increases), the velocity and spin interact: the spin becomes more positive, and the velocity becomes more negative. At point B, the spin actually becomes positive (i.e., the ball has lost all of its backspin and it is beginning to roll forward). Eventually, at around point C, the ball has lost all of its spin and velocity (due to frictional losses) and the ball becomes stationary.
Note how important the interaction between the spin and velocity is. The negative spin causes the velocity to be reduced much faster than would have happened if only friction was involved. This has the result that the ball can
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