Adjust object forward to force direction

Questions & Answers
1

I am pushing a cube around on a plane using AddRelativeForce. Expectedly, the cube does not change it's orientation when doing this. How can I orient the cube so that the positive Z is always facing the direction that the object is moving. In other words, if I created a direction bt taking the position of the last frame and the position of the current frame, how would I point the Z of the cube in that direction?

var xdirection : float;
var ydirection : float;

function Update(){
    //This in the user calibrated acceleration input code
    xdirection = Input.GetAxis ("Vertical") ;
    ydirection = Input.GetAxis ("Horizontal") ;
}

function FixedUpdate () {

    var relativeForce = Vector3(ydirection, 0, xdirection);
    rigidbody.AddRelativeForce (relativeForce);
}
2

something along the lines of this

var xdirection : float;
var ydirection : float;

function Update(){
    //This in the user calibrated acceleration input code
    xdirection = Input.GetAxis ("Vertical") ;
    ydirection = Input.GetAxis ("Horizontal") ;
}

function FixedUpdate () {

    var relativeForce = Vector3(ydirection, 0, xdirection);
    rigidbody.AddRelativeForce (relativeForce);
    if(relativeForce.magnitude > 0.01)
        rigidbody.rotation = Quaternion.LookRotation(relativeForce);
}