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) circles.add(makeCircle(R, S)); } for (let i = 0; i < 100; i++) { // lots of little circles circles.add(makeCircle(r, s * random(0, 1))); } } 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; } } }

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## Answer

It seems there are some bugs in the p5.play physics, in particular tunneling. Some big improvements already implemented, and more are to come. See https://github.com/molleindustria/p5.play/issues/214.