how to have a fixed movement using rigidbodies without animations

im making a game like crossy road and i want that my player can have a fixed movement

this is my script :

using System.Collections.Generic; using UnityEngine;

public class playermovement : MonoBehaviour {

 private bool is_grounded=true;


 private int rot;

 public Rigidbody rb;
 public float speed=10;

 public float jump_force;
 void Start () {
     
 }
 
 // Update is called once per frame
 void FixedUpdate () {

     if (Input.GetKeyDown (KeyCode.UpArrow)) {

         rot = 0;

         if (is_grounded == true) {
             jump ();
         }

         transform.eulerAngles = new Vector3(0,rot,0);

         rb.velocity = Vector3.forward * speed * Time.deltaTime;
     }
     if (Input.GetKeyDown (KeyCode.DownArrow)) {


         rot = 180;

         if (is_grounded == true) {
             jump ();
         }

         transform.eulerAngles = new Vector3(0,rot,0);


         rb.velocity = -Vector3.forward * speed * Time.deltaTime;


     }
     if (Input.GetKeyDown (KeyCode.RightArrow)) {

         rot = 90;
         if (is_grounded == true) {
             jump ();
         }


         transform.eulerAngles = new Vector3(0,rot,0);


         rb.velocity = Vector3.right * speed * Time.deltaTime;


     }
     if (Input.GetKeyDown (KeyCode.LeftArrow)) {
         rot = -90;

         if (is_grounded == true) {
             jump ();
         }

         transform.eulerAngles= new Vector3(0,rot,0);


         rb.velocity = Vector3.left * speed * Time.deltaTime;


     }
 }
 


 void OnCollisionStay(Collision others)
 {
     if (others.gameObject.tag == "Ground") {

         is_grounded = true;
     }
 }


 void jump()
 {
     //if (is_grounded == true) {
         rb.AddForce (new Vector3 (0, jump_force, 0) * Time.deltaTime);

         is_grounded = false;

 }

}