b2GravityController.prototype.Step = function (step) {
      var i = null;
      var body1 = null;
      var p1 = null;
      var mass1 = 0;
      var j = null;
      var body2 = null;
      var p2 = null;
      var dx = 0;
      var dy = 0;
      var r2 = 0;
      var f = null;
      if (this.invSqr) {
         for (i = this.m_bodyList;
         i; i = i.nextBody) {
            body1 = i.body;
            p1 = body1.GetWorldCenter();
            mass1 = body1.GetMass();
            for (j = this.m_bodyList;
            j != i; j = j.nextBody) {
               body2 = j.body;
               p2 = body2.GetWorldCenter();
               dx = p2.x - p1.x;
               dy = p2.y - p1.y;
               r2 = dx * dx + dy * dy;
               if (r2 < Number.MIN_VALUE) continue;
               f = new b2Vec2(dx, dy);
               f.Multiply(this.G / r2 / Math.sqrt(r2) * mass1 * body2.GetMass());
               if (body1.IsAwake()) body1.ApplyForce(f, p1);
               f.Multiply((-1));
               if (body2.IsAwake()) body2.ApplyForce(f, p2);
            }
         }
      }
      else {
         for (i = this.m_bodyList;
         i; i = i.nextBody) {
            body1 = i.body;
            p1 = body1.GetWorldCenter();
            mass1 = body1.GetMass();
            for (j = this.m_bodyList;
            j != i; j = j.nextBody) {
               body2 = j.body;
               p2 = body2.GetWorldCenter();
               dx = p2.x - p1.x;
               dy = p2.y - p1.y;
               r2 = dx * dx + dy * dy;
               if (r2 < Number.MIN_VALUE) continue;
               f = new b2Vec2(dx, dy);
               f.Multiply(this.G / r2 * mass1 * body2.GetMass());
               if (body1.IsAwake()) body1.ApplyForce(f, p1);
               f.Multiply((-1));
               if (body2.IsAwake()) body2.ApplyForce(f, p2);
            }
         }
      }
   }

这个if语句是为什么区分什么？