how get bow to add more force over time

hi I'm got the bow to shoot but I want it to add force over time like how it is in real life

public class Shoot : MonoBehaviour {

     [SerializeField]
     private GameObject arrow;
     [SerializeField]
     private Transform arrowSpan;
     [SerializeField]
     private float speed = 100f;
     [SerializeField]
     private float speedMutyPier = 2;
     [SerializeField]
     private float newSpeed;
 
     // Use this for initialization
     void Start () {
         
     }
     
     // Update is called once per frame
     void Update () {
         if (Input.GetButtonDown("Fire1"))
         {
           // here is trying to add force over time 
             speed * speedMutyPier = newSpeed; 
         }
         else if (Input.GetButtonUp("Fire1"))
         {
             Shooting();
         }
     }
 
     void Shooting()
     {
         GameObject Clone = (GameObject)Instantiate(arrow, arrowSpan.position, arrowSpan.rotation) as GameObject;
 
         Clone.GetComponent<Rigidbody>().AddForce(arrowSpan.forward * newSpeed);
     }
 }