class Ball {
  float x, y;          // position
  float vx=0, vy=0;    // velocity in the two directions   
  float r=5;           // radius
  float drag;          // multiplicative factor for velocity

  Ball(float x0, float y0, float drag)
  {
    x = x0; y = y0;
    this.drag = drag;
  }
  
  void draw()
  {
    ellipse(x,y,r*2,r*2);
  }

  void update()
  {
    // Damp according to drag
    vx *= drag;
    vy *= drag;
    // Move in the appropriate direction by the step size
    x += vx;
    y += vy;

    // Bounce, with frictionless walls
    if (x > width-r) { x=width-r; vx=-vx; }
    else if (x < r) { x=r; vx=-vx; }
    if (y > height-r) { y=height-r; vy=-vy; }
    else if (y < r) { y=r; vy = -vy; }
  }
}