class Ball { 
  /// add a force to the ball // 
  // Force = Mass * Acc //
  // Acceleration = Force / Mass //
  /////////////////////////////////
  Vector3D loc; 
  Vector3D vel;
  Vector3D acc; 
  Vector3D screenLoc; // location to store screen positions after translation. 
  //// add target positions to move to //// 
  Vector3D t;
  float mass;
  float max_vel;
  float w_top; // 'upper' width of ball 
  float w_base;
  Vector3D edge_top; // co-ordinates where the widht of the ball, in  given direction touches it's cirumference 
  Vector3D edge_base;
  //float w_deviation;
  ///////////////////////////////////////////////////////////////

  Ball (Vector3D loc_) {
    loc = loc_;
    t = new Vector3D (loc.x, loc.y); // store the target as it's original location // this could be updated ?? 
    //w = 4;//random (4);
    vel = new Vector3D (0, 0, 0);
    acc = new Vector3D (0, 0, 0);
    mass = 1.5;
    max_vel = 2;//.2;
    screenLoc  = new Vector3D (loc.x, loc.y, 0);
    edge_top = new Vector3D (0, 0);
    edge_base = new Vector3D (0, 0);
  } 

  // each ball has it's own force being added to it. 
  void addForce (Vector3D force) { 
    // FORCE = MASS * ACCELERATION;
    // ACCELERATION = FORCE  / MASS; 
    force.div (mass);
    acc.add (force);
  }
  ///////////////////////////////////
  ///////// add methods to update location according to velocity and acceleration ////////
  void update () { 
    vel.add(acc);
    vel.limit(max_vel);
    loc.add(vel);
    acc.setXYZ(0.0,0.0,0.0);
  } 
  //////

  //////////////////////////////////////////////
  void drawBall () { 
    ellipse (0, 0, w_base, w_base);
  }
  ////////////////////////////////////////////

}