physics unpredictability

If i use the following script on a cube, I expect the object to go up and then land in exactly the same spot.

it does not though.

its probably just the nature of using physics but i thought i would ask here anyway.

my physics material has everything set to zero.

function FixedUpdate () {

	if (Input.GetKey ("space")) {
		rigidbody.AddRelativeForce (0, 0, -200);
	}
	else{
		rigidbody.AddRelativeForce (0, 0, 500);
	}
}

Yes, it’s not 100% repeatable.

–Eric