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# Coeffiecient of Restitution ?

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Hi guys, Can I have two different restitutions, one for each object colliding ? When I compute the impulsive force, J, it just needs one. So what if I want to have a rubber ball (cor=1.25) hit a wooden ball (cor=0.75) ? I''m making believe the balls are particles. I was thinking of multiplying the restitutions and using that when I compute J, but I''m not sure if that''s logical. Thanks.

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it''s usually more complicated than that, unfortunetly. To be physcally accurate, you gotta have a table of materials, with a single restitution coefficient for each pair of meterials. Personnaly, if you are not worried about being that accurate, multiply them, or average them should be enough.

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Thanks Oliver,

Well, I''m going to make up my own materials because the game takes place in the far future but I was wondering how to come up with the values in a consistent way. For example:

Regular Ball hits Regular Wall = 1.0.
Bouncy Ball hits Regular Wall = 1.25.
Regular Ball hits Bouncy Wall = 1.25.
(with Cushy balls and walls: 0.75)

But what about a Bouncy Ball hitting a Bouncy Wall, know what I mean ?

Bouncy Ball hits Bouncy Wall
average = (1.25 + 1.25)/2.0 = 1.25 (same as hitting regular wall)
multiply = 1.25 * 1.25 = 1.56 (maybe better)

What do you think ?

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hmmm... i thought that the coefficient of restitution was always between 0 and 1... if its greater than 1 doesn''t that mean that if a ball hits a wall, it will actually gain speed after bouncing off it?

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I believe so. Conservation of momentum would be broken otherwise. Newton will be rising from his grave to kick the living crap out of you when that happens.

This is why, when multiplying them, you should always get a lower coefficient of restitution than any of the two. There is no definite caseon how to handle this. It really depends on the type of materials. really so, a bouncy ball should be 0.9f. Two bouncy ball would be 0.81f. But try to make a bouncy ball bounce on another bouncy ball, I''d think the CoR would be way below that. But sure, a bouncy ball of CoR 0.9f is a mighty bouncy ball already, probably not physcically achievable anyway, so you never know... Anyhow, something like multiplying the coeffs together seems logical.

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Coefficient of resitution for real-world materials is always < 1.0. (1.0 == perfect collision, which is nonexistent in the real world.)

Of course, with certain triggers, even in the real world you can simulate a better than perfect collision, e.g., one that adds kinetic energy to the system rather than remove energy. For example, if you have a spring that is compressed and when an object hits ground the spring is released by a trigger that activates on the collision, potential energy stored in the spring is converted to kinetic energy and added to the object causing it to bounce higher than it normally would even with a perfect collision.

Graham Rhodes
Senior Scientist
Applied Research Associates, Inc.

[edited by - grhodes_at_work on November 30, 2003 7:40:54 PM]

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You guys are right, I was playing around with it and got a big mess. I''m keeping it between 0.0 and 1.0 now.

Instead I added something like a pinball bumper that blasts the ball off faster when it gets hit, it works pretty nicely, I just gotta tweak it.

Thanks.

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