slow addforce

in my scene I have a ball that is thrown, so I added a addforce want you to slow down when it touches the ground its strength to a stop. I tried with rigidbody.sleep () but I do not like it stops abruptly. How can I do?

 public var speed : float;
 private var bar : float = 25;
 static  var myInt = 0;
 public var sfera : GameObject;
 public var coord : Vector3 = new Vector3 (0,5,5);
 public var forza : ForceMode;
 public var max : int = 100;
 public var min : int = 0; 
 static var count : int = 0;
 static var click : boolean = false;
 
 
 
 function Start(){
 
 
 }
 
 
 function Update(){
 
 
 
    if(/*Input.GetMouseButton(0) && count == 0*/ click == true && count == 0){
    
   
     
     speed += bar * Time.deltaTime;
     
     myInt = Mathf.FloorToInt(speed);
     
     
     
     if(myInt >= max){
     
     myInt = 100;
     
     }
     
     
     print(myInt);
     }
    
     if(Input.GetMouseButtonUp(0)){
   
      click = false;
      sfera.rigidbody.AddForce((coord*myInt)/2,forza);
      
      myInt = min;
      
      //count += 1;
   }
   gameObject.Find("Main Camera").SendMessage("Follow");
    
 }
 
 function OnMouseDown(){
 
   // if(click == false){
     
      click = true;
      
    //}
    }
    
 function OnMouseUp(){
 
     count += 1;
     lerp.pp = true;
     
 }
 
 function OnCollisionEnter(pavimento:Collision)
 
 {
 
 if(pavimento.gameObject.tag == "pavimento")
 
 {
     
     coord = new Vector3 (0,0.01,0.01);
     }
 
 
 }