acceleration

using UnityEngine;
using System.Collections;

public class ControlShip : MonoBehaviour {

    public int rotationSpeed = 75;
    public int movementspeed = 10;
    private int thrust = 5;

    bool isPKeyDown = false;
    float acceleration = .0f;

    Vector3 previousPosition = Vector3.zero;

    Rigidbody _rigidbody;

    // Use this for initialization
    void Start () {

        _rigidbody = GetComponent<Rigidbody>();
        Debug.Log("Acc Speed: " + thrust);
    }

    // Update is called once per frame
    void Update () {

        //var v3 = new Vector3(Input.GetAxis("Vertical"), Input.GetAxis("Horizontal"), 0.0f);
        //transform.Rotate(v3 * rotationSpeed * Time.deltaTime);
        //transform.position += transform.forward * Time.deltaTime * movementspeed; // this can break while calculations ... since its before calculating acceleration

        if (Input.GetKey(KeyCode.Z))
            transform.Rotate(Vector3.forward * rotationSpeed * Time.deltaTime);

        if ((isPKeyDown = Input.GetKey("p")))
        {
            float distance = Vector3.Distance(previousPosition, transform.position);
            acceleration = distance / Mathf.Pow(Time.deltaTime, 2);

            _rigidbody.AddRelativeForce(0f, 0f, thrust, ForceMode.Acceleration);
        }

        var v3 = new Vector3(Input.GetAxis("Vertical"), Input.GetAxis("Horizontal"), 0.0f);
        transform.Rotate(v3 * rotationSpeed * Time.deltaTime);
        transform.position += transform.forward * Time.deltaTime * movementspeed;

        previousPosition = transform.position;
    }

    void OnGUI()
    {
        if (isPKeyDown)
        {
            GUI.Label(new Rect(100, 100, 200, 200), "Acc Speed: " + acceleration);
        }
    }
}