Rigid body help

Ok i add a rigidbody to a tank with colliders but when i start the game the tank falls right under the map why is this

You did put a collider on the ground, right? Of course you did.

No i didn’t but i am making a game using the new demo that unity 3 comes with (boot camp) and i don’t know what the collider is called

What kind of collider are you using? Be sure it’s not set as a trigger since you want it to collide with the ground (assuming you do in fact have terrain or something under to collide with).