Unexpected loss of energy in frictionless perfectly elastic setting

[mbchang]

I'm using a modified version of the beachBalls example, with no friction, no air friction, and restitution of 1. When I let it run in the demo window, the balls eventually stop bouncing and come to a stop. I would expect them to keep bouncing indefinitely.

(function() {

    var World = Matter.World,
        Bodies = Matter.Bodies,
        Composites = Matter.Composites;

    Example.frictionlessBeachBalls = function(demo) {

        var engine = demo.engine,
            world = engine.world;

        // need random initialization
        var stack = Composites.stack(0, 100, 3, 1, 20, 0, function(x, y) {
            return Bodies.circle(x, y, 75, { restitution: 1, friction: 0, frictionAir: 0, frictionStatic: 0 });
        });
        World.add(world, stack);
    };

})();

I also tried setting the restitution of the world boundaries to 1 (the code below is modified from the Demo.reset, and that did not help.

        World.add(world, [
            Bodies.rectangle(400, -offset, 800.5 + 2 * offset, 50.5, { isStatic: true, restitution: 1 }),
            Bodies.rectangle(400, 600 + offset, 800.5 + 2 * offset, 50.5, { isStatic: true, restitution: 1 }),
            Bodies.rectangle(800 + offset, 300, 50.5, 600.5 + 2 * offset, { isStatic: true, restitution: 1 }),
            Bodies.rectangle(-offset, 300, 50.5, 600.5 + 2 * offset, { isStatic: true, restitution: 1 })
        ]);

I thought this might be an approximation issue when computing the effects of collisions and set high numbers for positionIterations, velocityIterations, and constraintIterations.

        demo.engine.positionIterations = 1000
        demo.engine.velocityIterations = 1000
        demo.engine.constraintIterations = 1000

This did not help either.

Is this loss of energy due to numerical approximation errors, or can this energy loss be explicitly controlled? At the moment beyond adjusting restitution and friction I am unable to find ways to make energy conservation perfect.

[liabru]

See #21. Basically the linear momentum is being converted to angular momentum on collision (because circles are approximated at the moment).

[liabru]

Hopefully my last post helped you resolve your issue, so closing this. Feel free to reply if you need more help.

[wmike1987]

I was trying something similar to the original poster. Just a ball bouncing on a static body.

var ball = Bodies.circle(100, 250, 50, { inertia: Infinity, restitution: 1, friction: 0, frictionAir: 0, frictionStatic: 0});

Even with inertia set to Infinity, the ball loses energy and bounces closer and closer to the ground, although interestingly never fully stopping. It seemingly bounces forever very close to the ground. Am I missing something?

[liabru]

How quickly does it lose energy? I realise conservation isn't exactly perfect at the moment, which is a bit of a problem, but hopefully it takes a while? Can you show a jsfiddle?

[wmike1987]

It loses it noticeably quick.

https://jsfiddle.net/vd7d25pu/7/

[liabru]

Thanks for the example. It's pretty quick indeed, I will investigate this. I expect it may be due to floating point errors but it does seem a little quick even for that.

[wmike1987]

Take a look at this jsfiddle:

https://jsfiddle.net/vd7d25pu/10/

Notice that the ball does appear to bounce forever here. The only difference is that the 'ground' rectangle's height is set to 1. I stumbled upon this accidentally.

[kujma10-zz]

I have the same problem with many balls (gravity turned off)
Here is my jsfiddle example: https://jsfiddle.net/vd7d25pu/11/

I considered @wmike1987 's comment above and set 1 height to borders. However balls are still losing the force and not moving forever, they stop moving after some time. Probably an energy loss is after ball collisions.

[kujma10-zz]

One thing I noticed: if I increase mass of the balls (and force proportionally) then it lasts longer but still not infinite.