# Two circles collision math problem

**URL:** <https://forum.kirupa.com/t/two-circles-collision-math-problem/233302>\
**Category:** programming\
**Created:** [July 22, 2007, 5:27pm UTC](https://forum.kirupa.com/t/two-circles-collision-math-problem/233302 "2007-07-22T17:27:51Z")\
**Posts on this page:** 1\
**Page:** 1

<div class="post-metadata">

**Author:** ![John\_NU](https://avatars.discourse-cdn.com/v4/letter/j/a8b319/32.png) [@John\_NU](https://forum.kirupa.com/u/John_NU)\
**Post date:** [July 22, 2007, 5:27pm UTC](https://forum.kirupa.com/t/two-circles-collision-math-problem/233302/1 "2007-07-22T17:27:51Z")

</div>

I am currently developing a game that has balls of different radii and mass. My code works properly when all balls are of the same mass; but if i introduce a ball with a greater mass, the results are unexpected and incorrect. Please help me find my flaw.

the AS:  
[ballHelp.as](http://jnuwebsites.com/help/ballHelp.as)

the swf:  
[Ball.swf ( [URL=“http://jnuwebsites.com/help/Ball.html”]swf in html](http://jnuwebsites.com/help/Ball.swf) )

The problem is that most of the time the reaction to collision is accurate, but occasionally the two balls will ‘stick’ together. This only seems to happen when one has a larger mass (i think. Here is a [swf of two equal mass balls](http://jnuwebsites.com/help/BallSimple.swf) for comparison.

Below is the main collision function which is triggered if the distance between the two balls is greater than the sum of thier radii:

```auto

        public function doPhysicsTransform(b1:Ball,b2:Ball) {
            var bounce = -.9;
            var distx = b2.x-b1.x;
            var disty = b2.y-b1.y;
            var distSQ = distx*distx+disty*disty;

            var angle = Math.atan2(disty, distx);
            var dist = Math.sqrt(distSQ);

            var cosa = Math.cos(angle);
            var sina = Math.sin(angle);

            var b1vxp = b1.dx*cosa+b1.dy*sina;
            var b1vyp = b1.dy*cosa-b1.dx*sina;
            var b2vxp = b2.dx*cosa+b2.dy*sina;
            var b2vyp = b2.dy*cosa-b2.dx*sina;
            var P = b1vxp*b1.m+b2vxp*b2.m;
            var V = b1vxp-b2vxp;
            b1vxp = (P-b2.m*V)/(b1.m+b2.m);
            b2vxp = V+b1vxp;

            b1.dx = (b1vxp*cosa-b1vyp*sina)*-bounce;
            b1.dy = (b1vyp*cosa+b1vxp*sina)*-bounce;
            b2.dx = (b2vxp*cosa-b2vyp*sina)*-bounce;
            b2.dy = (b2vyp*cosa+b2vxp*sina)*-bounce;

            var diff = (b1.radius+b2.radius)-dist;
            b1.x -= Math.cos(angle)*diff/2;
            b1.y -= Math.sin(angle)*diff/2;
            b2.x += Math.cos(angle)*diff/2;
            b2.y += Math.sin(angle)*diff/2;

        }

```

This code was derived from an [old discussion on billiards physics](http://www.flashkit.com/board/showthread.php?threadid=281136&perpage=10&pagenumber=1). Again any help is much appreciated.

That last section of code is to move the balls apart so that their edges are touching rather than overlaping. I think the issue might be in there somewhere, but i am not certain what the problem is. Please help, thanks.