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    <p>I'm the one who wrote the original ball bounce code you referenced. If you <a href="http://code.google.com/p/simucal-projects/source/browse/ballbounce/trunk/Ball.java" rel="nofollow noreferrer">download</a> and try out that code, you can see it works fine. </p> <p>The following code is correct (the way you originally had it):</p> <pre><code>// collision impulse float i = (-(1.0f + RESTITUTION_CONSTANT) * vn) / (im1 + im2); CGPoint impulse = ccpMult(mtd, i); </code></pre> <p>This is very common physics code and you can see it nearly exactly implemented like this in the following examples: </p> <ul> <li><a href="http://www.gamedev.net/community/forums/topic.asp?topic_id=531478" rel="nofollow noreferrer">Find collision response of two objects - GameDev</a> </li> <li><a href="http://www.flipcode.com/archives/3D_Pong_Collision_Response.shtml" rel="nofollow noreferrer">3D Pong Collision Response</a> </li> <li><a href="http://www.gamedev.net/community/forums/topic.asp?topic_id=497363" rel="nofollow noreferrer">Another ball to ball collision written in Java</a> </li> </ul> <p>This is correct, and it ~isn't~ creating a CoR over 1.0 like others have suggested. This is calculating the relative impulse vector based off mass and Coefficient of Restitution.</p> <p>Ignoring friction, a simple 1d example is as follows: </p> <pre><code>J = -Vr(1+e) / {1/m1 + 1/m2} </code></pre> <p>Where e is your CoR, Vr is your normalized velocity and J is a scalar value of the impulse velocity.</p> <p>If you plan on doing anything more advanced than this I suggest you use one of the many physics libraries already out there. When I used the code above it was fine for a few balls but when I ramped it up to several hundred it started to choke. I've used the Box2D physics engine and its solver could handle more balls and it is much more accurate.</p> <p>Anyway, I looked over your code and at first glance it looks fine (it is a pretty faithful translation). It is probably a small and subtle error of a wrong value being passed in, or a vector math problem. </p> <p>I don't know anything concerning iPhone development but I would suggest setting a breakpoint at the top of that method and monitoring each steps resulting value and finding where the blow-up is. Ensure that the MTD is calculated correctly, the impact velocities, etc, etc until you see where the large increase is getting introduced.</p> <p>Report back with the values of each step in that method and we'll see what we have.</p>
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