Why -Physics.gravity * mass doesn't cancel the gravity force?

I have a rigidBody with gravity checked.Naturaly without any force,it should fall to the ground.and when you add a force equal to gravity in magnitude but in a negative direction,it should not move any more,(at least it should NOT have acceleration).
but surprisingly it does!
can some body help me out? tnx in advance :slight_smile:

void Update ()
{
	Vector3 t = Physics.gravity * rigidbody.mass;
	rigidbody.AddForce (-t);
}

You have to apply forces in FixedUpdate. But why applying an acceleration and the opposite force ta the same time?