How to get PhysicsWorld's instance in Lua?

I’ve got no idea how to get PhysicsWorld’s instance in Lua?

cc.Director:getInstance():getRunningScene():getPhysicsWorld()

Please refer to PhysicsTest of lua-test project in cocos2d-x framework.

I’ve already resolved this problem,I can get the instance with “cc.PhysicsBody:getWorld()”.But,thank you all the same!