this.move = function(dt)
      {
        var t, a, c, dtdt; 
        
        dtdt = dt * dt; 
        
        a = this.force.getX() / this.mass;
        c = this.cur.getX(); 
        t = (2.0 - this.friction) * c - (1.0 - this.friction) * this.prev.getX() + a * dtdt;
        this.prev.setX(c); 
        this.cur.setX(t);
              
        a = this.force.getY() / this.mass;
        c = this.cur.getY(); 
        t = (2.0 - this.friction) * c - (1.0 - this.friction) * this.prev.getY() + a * dtdt;
        this.prev.setY(c); 
        this.cur.setY(t);
      }
      this.setFriction = function(friction)
      {
        this.friction = friction; 
      }
      this.getVelocity = function()
      {
        var cXpX, cYpY; 
        
        cXpX = this.cur.getX() - this.prev.getX(); 
        cYpY = this.cur.getY() - this.prev.getY();
        
        return cXpX * cXpX + cYpY * cYpY;  
      }
      this.draw = function(ctx, scaleFactor)
      {
        ctx.lineWidth = 2; 
        ctx.fillStyle = '#000000'; 
        ctx.strokeStyle = '#000000'; 
        ctx.beginPath(); 
        ctx.arc(this.cur.getX() * scaleFactor, 
                this.cur.getY() * scaleFactor, 
                4.0, 0.0, Math.PI * 2.0, true); 
        ctx.fill(); 
      }
    }