Question

In this program I simulate the gravity. All works but when there is no more movement for the ball it keep bouncing 1-5 pixels depending on the gravity value I set. How could i stop the ball when the energy is lost? I want the xSpeed to become 0 and the ball to stay on a fix position.

Edit: The gravity variate from 1 to 100. The user can change the gravity.

energyLoss = 0.9 and dt = 0.2

// right and left wall collision
        if (x + xSpeed > this.getWidth() - radius - 1) {
            x = this.getWidth() - radius - 1;
            xSpeed = -xSpeed;
        } else if (x + xSpeed < 0 + radius) {
            x = 0 + radius;
            xSpeed = -xSpeed;
        } else
            x += xSpeed;

        if (y == this.getHeight() - radius - 1) {

        }

        if (y > this.getHeight() - radius - 1) {
            y = this.getHeight() - radius - 1;
            ySpeed *= energyLoss;
            ySpeed = -ySpeed;

            // friction with the ground
            xSpeed *= xFriction;
            if (Math.abs(xSpeed) < .4)
                xSpeed = 0;

        } else {
            ySpeed += gravity * dt; // velocity formula
            y += ySpeed * dt + .5 * gravity * dt * dt; // position formula
        }

        repaint();

Answer 1

The simplest way to fix this problem is to detect a special case where the ball is on the wall with 0 (or very small) Y velocity. This is basically what DwB is suggesting.

However, you then need to go further and make sure that you stop applying gravity when you’re in that situation.

Something like this:

// right and left wall collision
    if (x + xSpeed > this.getWidth() - radius - 1) {
        x = this.getWidth() - radius - 1;
        xSpeed = -xSpeed;
    } else if (x + xSpeed < 0 + radius) {
        x = 0 + radius;
        xSpeed = -xSpeed;
    } else
        x += xSpeed;

    if (y == this.getHeight() - radius - 1 
        && ySpeed == 0) { // Check speed too here!
        // Do nothing for Y.
        // friction with the ground
        xSpeed *= xFriction;
        if (Math.abs(xSpeed) < .4)
            xSpeed = 0
    }
    else if (y > this.getHeight() - radius - 1) {
        y = this.getHeight() - radius - 1;
        ySpeed *= energyLoss;
        if (Math.abs(ySpeed) < SomeMinimumValue)
            ySpeed = 0;
        else // invert speed, i.e. change direction.
            ySpeed = -ySpeed;

        // friction with the ground
        xSpeed *= xFriction;
        if (Math.abs(xSpeed) < .4)
            xSpeed = 0;

    } else {
        ySpeed += gravity * dt; // velocity formula
        y += ySpeed * dt + .5 * gravity * dt * dt; // position formula
    }

    repaint();