Bouncing ball stops bouncing?

Question

I could not find how to ask so there is a demo at the link. I am pretty sure this is a simple question but I can not seem to find why.

My question is: Why the ball stops bouncing? Is it about something like rounding in multiplications or divisions etc...

   void move() {
        PVector v0 = v;
        PVector dv = PVector.mult(a, deltaT);
        v.add(dv);

        PVector d = PVector.add(
            PVector.mult(v0, deltaT),
            PVector.mult(dv, deltaT * 0.5));

        move(d.x, d.y);
    }


    void move(float dx, float dy) {
        p.x += dx;
        p.y += dy;

        if (p.x > width - r) {
            p.x = width - r;
            v.x = -v.x;
        }
        if (p.x < r) {
            p.x = r;
            v.x = -v.x;
        }
        if (p.y > height - r) {
            p.y = height - r;
            v.y = -v.y;
        }
        if (p.y < r) {
            p.y = r;
            v.y = -v.y;
        }
    }

Answer 1

It's an issue with this part of the code:

if (p.y > height - r) {
   p.y = height - r;
   v.y = -v.y;
}

Imagine that height is 600 and r is 10. When the ball hits the bottom (p.y > (600 - 10)) it gets reset to (600-10). But the ball can hit the bottom below that: p.y can be higher than height, leading to the issue you're having. The correct code in this case would then be:

if (p.y > height - r) {
   p.y = (p.y - height) + (height - r);
   v.y = -v.y;
}

Being (p.y - height) what you're losing each time the ball bounces.

Hope this helps.

Answer 2

There is a multiplication by a variable that is < 0 in every Move of Ball object. Its the PVector a; of Ball object which initially gets the value of gravity (PVector gravity = new PVector(0, 0.0025);) in the setup()

ball.a = gravity;

Then in every Move

 PVector dv = PVector.mult(a, deltaT);

Eventually this leads to a PVector dv value of zero.

if you change the

PVector gravity = new PVector(0, 0.0025);

to

PVector gravity = new PVector(0, 0.025);

you will see a stronger effect of gravity.