function PointMass(cx, cy, mass)
    {
      this.cur = new Vector(cx, cy); 
      this.prev = new Vector(cx, cy); 
      this.mass = mass; 
      this.force = new Vector(0.0, 0.0); 
      this.result = new Vector(0.0, 0.0); 
      this.friction = 0.01; 
      
      this.getXPos = function()
      {
        return this.cur.getX(); 
      }
      this.getYPos = function()
      {
        return this.cur.getY(); 
      }
      this.getPos = function()
      {
        return this.cur; 
      }
      this.getXPrevPos = function()
      {
        return this.prev.getX(); 
      }
      this.getYPrevPos = function()
      {
        return this.prev.getY(); 
      }
      this.getPrevPos = function()
      {
        return this.prev; 
      }
      this.addXPos = function(dx)
      {
        this.cur.addX(dx); 
      }
      this.addYPos = function(dy)
      {
        this.cur.addY(dy); 
      }
      this.setForce = function(force)
      {
        this.force.set(force); 
      }    
      this.addForce = function(force)
      {
        this.force.add(force); 
      }
      this.getMass = function()
      {
        return this.mass; 
      }
      this.setMass = function(mass)
      {
        this.mass = mass; 
      }