On collision enter Add rigidbody?

How add a Rigidbody on enter collision?

var tanque : GameObject;

function OnCollisionEnter(collision : Collision) {

tanque.AddComponent(Rigidbody);

tanque.rigidbody.mass = 500;

it needs to be

tanque.AddComponent("Rigidbody");

as its looking for the class name, which is a string.

Thank You!