class Spring { 
  float stiffness;// = 0.5; // alters the force of the spring
  float damping;// = 0.5;// add some friction 
  float mass;// = 2.0;
  float springLength = MAX_DIST;//30;
  //////////////
  Spring () { 

  }
  Spring (float s, float d, float m) { 
    stiffness = s; 
    damping = d; 
    mass = m;

  }
  //////////////
  // 1.input the CURRENT target position to 'spring towards' (i.e. the previous ball in array) 
  // 2.calculate distnace (dx, dy) relative to the  current postion of the ball 
  // 3.calculate angle 
  // 4.calculate where the object SHOULD move to (using springLength) (new target postion to maintain a distance from)
  // 5.calculate force and acceleration
  // 6. 
  //
  // input 2 ball objects  - rather than just their locations - so we can get their loc and vel////
  void applySpring (Ball ball, Ball prevball, boolean recalc) { // tx, ty
    // calculate target (using springLength); 
    // find distance between the locs of two ball locs
    Vector3D d = ball.loc.sub (ball.loc, prevball.loc);
    float distance = d.magnitude(); 
    //println(int (d.magnitude()));
    // find angle 
    float angle = d.heading2D ();
    //println (degrees (angle));
    ///////////////////////////////////////////
    /// recalculate target positions 
    float tx; 
    float ty;
    if (recalc) {
      tx = prevball.loc.x+ cos (angle) * springLength; 
      ty = prevball.loc.y + sin (angle) * springLength;
    } 
    else { 
      tx = prevball.loc.x; 
      ty = prevball.loc.y;
    }
    // calculate force 
    float xforce = stiffness * (tx - ball.loc.x);
    float yforce = stiffness * (ty - ball.loc.y);
    /// use mass to influence acceleration /////
    float xAcc = xforce / mass; 
    float yAcc = yforce / mass; 
    // velocity = friction * (velocity + springforce); //
    ball.vel.x = damping * (ball.vel.x + xAcc); 
    ball.vel.y = damping * (ball.vel.y + yAcc); 
    //xvel = damping * (xvel + xAcc); 
    //yvel = damping * (yvel + yAcc); 
    // update location //
    ball.loc.add (ball.vel);
  }
  /////////////////
  void moveBacktoTarget (Vector3D loc, Vector3D targ) { 
    // find distance 
    float distance = loc.distance (targ, loc);
    //println (distance);

  }

}