unity forces

using System.Collections;
using System.Collections.Generic;
using UnityEngine;

public class PlayerController : MonoBehaviour {

    public Rigidbody rb;
    [SerializeField] private float force;
    [SerializeField] private Transform cameraPos;
    [SerializeField] private Vector3 difference;
    // Use this for initialization
    void Start () {
        rb = GetComponent<Rigidbody>();

    }

    // Update is called once per frame
    void Update ()
    {
        if (Input.GetKey("d"))
        {
            rb.AddForce(force * Time.deltaTime, 0f, 0f, ForceMode.Impulse);
        }
        if(Input.GetKey("a"))
        {
            rb.AddForce(-force * Time.deltaTime, 0f, 0f, ForceMode.Impulse);
        }
        if (Input.GetKey("w"))
        {
            rb.AddForce(0f, 0f, force * Time.deltaTime, ForceMode.Impulse);
        }
        if (Input.GetKey("s"))
        {
            rb.AddForce(0f, 0f, -force * Time.deltaTime, ForceMode.Impulse);
        }

    }
}