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Bob Jewett
Northern California

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10-25-2007, 08:38 AM

Quote:
Originally Posted by Patrick Johnson
Can you describe what "elasticity" is, how it makes balls bouncier and what that means generally for ball/ball interactions? I've only heard about it in pieces and haven't been able to add it all up.

Thanks,

pj
chgo
Elasticity is how well a ball will transfer its energy to another ball. On a stop shot, we hope for the object ball to leave with 100% of the cue ball's incoming speed. If the balls are inelastic, the object ball will get only part of the cue ball's speed. A result of this, required by the laws of conservation of energy and momentum, is that the cue ball will retain some of its initial velocity, and be going forward slightly after the collision. This action is much more visible with ivory balls which are much less elastic than pool balls.

One way to measure elasticity is to bounce a ball off a very hard, heavy object, like a steel block. (This gives a situation just like two balls running at each other with the same speed, since neither ball will penetrate the point where they collide, just as the ball will not penetrate (significantly) the steel block.) Neglecting air resistance, the bounce height divided by the starting height gives the square of the "coefficient of restitution" of the collision, and gives a ratio of speeds before and after the collision. Wikipedia has several sections on this stuff.

If a cue ball is inelastic, it will follow well and draw poorly even though it is the same mass as the object ball.


Bob Jewett
SF Billiard Academy
  
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