public void resolveCollision(Ball ball)
    {

        // get the mtd
        Vector2d delta = (position.subtract(ball.position));
        float r = getRadius() + ball.getRadius();
        float dist2 = delta.dot(delta);

        if (dist2 > r*r) return; // they aren't colliding


        float d = delta.getLength();

        Vector2d mtd;
        if (d != 0.0f)
        {
            mtd = delta.multiply(((getRadius() + ball.getRadius())-d)/d); // minimum translation distance to push balls apart after intersecting

        }
        else // Special case. Balls are exactly on top of eachother.  Don't want to divide by zero.
        {
            d = ball.getRadius() + getRadius() - 1.0f;
            delta = new Vector2d(ball.getRadius() + getRadius(), 0.0f);

            mtd = delta.multiply(((getRadius() + ball.getRadius())-d)/d);
        }

        // resolve intersection
        float im1 = 1 / getMass(); // inverse mass quantities
        float im2 = 1 / ball.getMass();

        // push-pull them apart
        position = position.add(mtd.multiply(im1 / (im1 + im2)));
        ball.position = ball.position.subtract(mtd.multiply(im2 / (im1 + im2)));

        // impact speed
        Vector2d v = (this.velocity.subtract(ball.velocity));
        float vn = v.dot(mtd.normalize());

        // sphere intersecting but moving away from each other already
        if (vn > 0.0f) return;

        // collision impulse
        float i = (-(1.0f + Constants.restitution) * vn) / (im1 + im2);
        Vector2d impulse = mtd.multiply(i);

        // change in momentum
        this.velocity = this.velocity.add(impulse.multiply(im1));
        ball.velocity = ball.velocity.subtract(impulse.multiply(im2));

    }