[LayaAirIDE 2.0]新手请问下如何开启Laya3D的physics

/**

	*@inheritDoc

	*/

	__proto._onAdded=function(){

		var physics3D=Laya3D._physics3D;

		var btColObj=new physics3D.btCollisionObject();

		btColObj.setUserIndex(this.id);

		btColObj.forceActivationState(5);

		var flags=btColObj.getCollisionFlags();

		if ((this.owner).isStatic){

			if ((flags & 2)> 0)

				flags=flags ^ 2;

			flags=flags | 1;

			}else {

			if ((flags & 1)> 0)

				flags=flags ^ 1;

			flags=flags | 2;

		}

		btColObj.setCollisionFlags(flags);

		this._nativeColliderObject=btColObj;

		_super.prototype._onAdded.call(this);

	}

physics3D是undefine，是需要在哪儿初始化一下物理吗