# incorrect bouncing behaviour in p5js play library

I’m using the `play` library: https://molleindustria.github.io/p5.play/

I’m doing a simple bouncing balls demo, with one large ball whose mass in `play` is given by \$pi r^2\$. The basic physics looks right to me. But after a few seconds, the large ball starts to jump large distances when struck by a small ball.

Here’s my complete code (EDIT, also pasted below): https://editor.p5js.org/jmmcd/sketches/BMPtPY098

It seems like a bug in the collision/bounce code. But my code is conceptually the same as https://molleindustria.github.io/p5.play/examples/index.html?fileName=collisions4.js, which doesn’t have this problem.

```// Inspired by Drescher's discussion of the
// arrow of time in "Good and Real"

// there is a bug I can't fix: soon after the animation starts,
// the large circle starts jumping as if being displaced or as
// if a collision goes wrong

let circles;

function setup() {
createCanvas(400, 400);
r = 5; s = 1; // radius and speed of small circles
R = 50; S = 2; // radius and speed of large circles
circles = new Group;
for (let i = 0; i < 1; i++) {
// big circle(s)
}
for (let i = 0; i < 100; i++) {
// lots of little circles
}
}

function makeCircle(r, s) {
let col = [random(255), random(255), random(255)];
c = createSprite(random(width), random(height), r, r);
c.draw = function() {
fill(col);
ellipse(0, 0, r, r)
}
c.setCollider("circle");
c.setSpeed(s, random(0, 360));
c.mass = PI * r * r;
c.scale = 1;
return c;
}

function draw() {
background(220);
circles.bounce(circles);
bounceGroupWalls(circles);
for (let c of circles) {
c.debug = mouseIsPressed;
}
drawSprites();
status();
}

function status() {
textSize(12);
let s = 0;
for (let c of circles) {
s += c.getSpeed() * c.mass;
}
s = s.toFixed(0);
text("Momentum " + s, 30, 30);
}

function bounceGroupWalls(g) {
//all sprites in group g bounce at the screen edges
for (let s of g) {
if(s.position.x <= 0 || s.position.x >= width) {
s.velocity.x *= -1;
}
if(s.position.y <= 0 || s.position.y >= height) {
s.velocity.y *= -1;
}
}
}
```