class mass
{
  position p;
  vector velocity;  
  float massAmount;
  
  mass(position pt)
  {
    p=new position(pt); 
    velocity=new vector(p,0,0);   
  }
  void update()
  {
    if(useGravity)
      doGravity();  
    doFriction();
    doVelocity();
    doBorders();      
  }  
  void doGravity()
  {
    velocity=velocity.add(GRAVITY_VECTOR);
  }
  void doFriction()
  {
    velocity.m*=FRICTION_FORCE;
  }
  void doVelocity()
  {
    if(useRK)
      p=p.displace(velocity);  
    else
      p=p.displace(velocity);
  }
  void doBorders()
  {
    boolean collide=false;
    if(p.x<10)
    {
      p.x=10;
      collide=true;
    }  
      
    else
      if(p.x>width-10)
      {
        p.x=width-10;
        collide=true;
      } 
    if(p.y<50)
    {
      p.y=50;
      collide=true;
    }
    else
      if(p.y>height-50)
      {
        p.y=height-50;
        collide=true;
      }
    if(collide)
      velocity.m*=.7;
  }
  float x()
  {
    return p.x;  
  }
  float y()
  {
    return p.y;
  }
}