Billiards and Physics

This java applet show you how to use the law of reflection (for optics) in playing pool.

A ball bouncing off the bank of a pool table behaves like a light ray reflecting off a mirror.

There are two colored balls; red and blue.

How to play?

Click LEFT mouse button within the red ball and drag the mouse Click the RIGHT mouse button to change the initial position for the red ball.

More information will be provided If you select level1 or level2.

Try it and find out the physics behind the game.

(For collision between two balls, please check out 2D Collision java applet)

Subject: RE: Billiards and Physics
Date: Mon, 5 Mar 2001 09:38:53 -0500
From: John Botsford <>
To: 'Fu-Kwun Hwang' <>

    John Botsford wrote:

    >  The law of reflection can only be used in billiards to
    >  approximate the angle of reflection for a billiard ball.  Since
    >  the rails of a pool table are rubber, the ball will sink into
    >  the cushion to varying degrees depending on the velocity of the
    >  ball.  This amount that a ball sinks into the cushion changes
    >  the angle of reflection.  For instance, the faster a ball is
    >  moving when it hits the rail, the sharper the angle of
    >  reflection will be.  The law of reflection is a close
    >  approximation only if the ball is moving relatively slowly.  The
    >  other thing that affects the angle of reflection is the spin on
    >  a ball.  In the case of the cue ball, spin is often purposely
    >  mparted to the ball by striking it off center in order to change
    >  where the cue ball will end up.  As for object balls, spin is
    >  imparted to them by contact with other objects (balls and
    >  rails).  This spin also changes the angle of reflection.
     Yes. It is true that the law of reflection is only an approximation of what really happened!
     The effect of rubber and the spining effect also can be simulated
     if we know the spring constant and apply conservation of momentum,
     energy and angular momentum. But that would be too complicate to
    the students who just learn physics.
     The purpose of our animation is for teaching. That is why I
     simplify the problem and the result are still very similar to what
     really happened.  Thank you for your comments. Could I add your
     comments to the end of the web pages?

Your suggestions are highly appreciated! Please click
Author¡GFu-Kwun Hwang, Dept. of physics, National Taiwan Normal University
Last modified : 

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