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POBall to Ball Collision - Gains significant velocity upon collision
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copied!<p>I implemented the code from the question "<a href="https://stackoverflow.com/questions/345838/ball-to-ball-collision-detection-and-handling">Ball to Ball Collision - Detection and Handling</a>" in Objective-C. However, whenever the balls collide at an angle their velocity increases dramatically. All of the vector math is done using <a href="http://code.google.com/p/cocos2d-iphone/source/browse/trunk/cocos2d/Support/CGPointExtension.h?spec=svn806&r=806" rel="nofollow noreferrer">cocos2d-iphone</a>, with the header CGPointExtension.h. What is the cause of this undesired acceleration?</p> <p>The following is an example of increase in speed: </p> <p><strong>Input:</strong><br> mass == 12.56637<br> velocity.x == 1.73199439<br> velocity.y == -10.5695238 </p> <p>ball.mass == 12.56637<br> ball.velocity.x == 6.04341078<br> ball.velocity.y == 14.2686739</p> <p><strong>Output:</strong><br> mass == 12.56637<br> velocity.x == 110.004326<br> velocity.y == -10.5695238 </p> <p>ball.mass == 12.56637<br> ball.velocity.x == -102.22892<br> ball.velocity.y == -72.4030228</p> <pre><code>#import "CGPointExtension.h" #define RESTITUTION_CONSTANT (0.75) //elasticity of the system - (void) resolveCollision:(Ball*) ball { // get the mtd (minimum translation distance) CGPoint delta = ccpSub(position, ball.position); float d = ccpLength(delta); // minimum translation distance to push balls apart after intersecting CGPoint mtd = ccpMult(delta, (((radius + ball.radius)-d)/d)); // resolve intersection -- // inverse mass quantities float im1 = 1 / [self mass]; float im2 = 1 / [ball mass]; // push-pull them apart based off their mass position = ccpAdd(position, ccpMult(mtd, (im1 / (im1 + im2)))); ball.position = ccpSub(ball.position, ccpMult(mtd, (im2 / (im1 + im2)))); // impact speed CGPoint v = ccpSub(velocity, ball.velocity); float vn = ccpDot(v,ccpNormalize(mtd)); // sphere intersecting but moving away from each other already if (vn > 0.0f) return; // collision impulse float i = (-(1.0f + RESTITUTION_CONSTANT) * vn) / ([self mass] + [ball mass]); CGPoint impulse = ccpMult(mtd, i); // change in momentum velocity = ccpAdd(velocity, ccpMult(impulse, im1)); ball.velocity = ccpSub(ball.velocity, ccpMult(impulse, im2)); } </code></pre>

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