Compare rigidbody.centerofMass with go.rigidbody.centerofMass ?

hello, sorry about my English. First, this is my code:

  Rigidbody rigid;
     void start()
     {
     rigid = gameObject.AddComponent<Rigidbody>();
     rigid.centerOfMass = Vector3.zero;
     }

 void Start()
 {
 gameObject.rigidbody.centerOfMass = Vector3.zero;
 }

First, i think their are same, but when i used them. The result of first code is not my expected. Please help me to distinguish them. Thanks.