class PhysBall {
  float x, y;          // position
  float vx=0, vy=0;    // velocity in the two directions   
  float r=10;          // radius
  color c=color(255);  // color
  float gravity=0.1;   // the amount of acceleration
  float drag=0.99;     // multiplicative factor for velocity
  float frict=0.75;   // multiplicative factor, only when bounce
  
  // Initialize a Ball at position (x0,y0)
  PhysBall(float x0, float y0, color c0)
  {
    x = x0; 
    y = y0;
    c = c0;
  }

  void draw()
  {
    fill(c);
    stroke(red(c)/2,green(c)/2,blue(c)/2);
    ellipse(x,y,r*2,r*2);
  }

  void updateVelocity() {
    // Accelerate according to gravity
    vy += gravity;
    // Now damp according to drag
    vx *= drag;
    vy *= drag;
  }
  
  void bounce() {
    if (x > width-r || x < r) { 
      x = constrain(x,r,width-r); 
      vx = -vx; 
      vy *= frict;   // damp
    }
    if (y > height-r || y < r) { 
      y = constrain(y,r,height-r); 
      vy = -vy; 
      vy *= frict;   // damp
    }
  }

  void update()
  {
    // update the velocity
    updateVelocity();
    
    // Move in the appropriate direction by the step size
    x += vx;
    y += vy;
    
    // bounce
    bounce();
  }
}