public Rigidbody rb;
public float forwardForce = 2000f;
public float sidewaysForce = 500f;

void Start () {

}

// Update is called once per frame
void FixedUpdate () {
    rb.AddForce(0, 0, forwardForce * Time.deltaTime);

    if (Input.GetKey("d")) {
        rb.AddForce(sidewaysForce * Time.deltaTime, 0, 0,);
    }
    if (Input.GetKey("a")) {
       rb.AddForce(-sidewaysForce * Time.deltaTime, 0, 0,);
    }
}