I can't fix this error!

I'm getting an error: paddleScript.cs(42,31): error CS0200: Property or indexer `UnityEngine.GameObject.rigidbody' cannot be assigned to (it is read only)

Here is my script:

using UnityEngine;

using System.Collections;

 public class paddleScript : MonoBehaviour {
     
      float paddleSpeed = 10f;
      public GameObject ballPrefab;
      private Vector3 ballPosition;
      private Quaternion ballRotation;
     
     
      GameObject attachedBall = null;
     
     // Use this for initialization
     void Start () {
         
         
     SpawnBall();    
     
     }
     
     void SpawnBall() {
          Vector3 ballPosition;
          attachedBall = ballPrefab;
              
          
         //Spawns the ball, duh!
         
         ballPosition = transform.position + new Vector3(0, 1f, 0);
         Quaternion ballRotation = Quaternion.identity;    
        
         
         attachedBall = (GameObject)Instantiate( ballPrefab, ballPosition, ballRotation );
     }    
     
     // Update is called once per frame
     void Update () {
     
         transform.Translate ( paddleSpeed * Time.deltaTime * Input.GetAxis ( "Horizontal" ), 0, 0 );
         
         if( attachedBall ) {
          attachedBall.rigidbody = transform.position + new Vector3(0, .75f, 0);    
         }    
         if( Input.GetButtonDown( "Jump" ) ){
            //Fire the ball!
            if( attachedBall ) {
            attachedBall.rigidbody.isKinematic = false;        
            attachedBall.rigidbody.AddForce(0, 300f, 0);
            attachedBall = null;        
             
         }
     }
 }        
     
     
     void  OnCollisionEnter( Collision col ) {
         foreach (ContactPoint contact in col.contacts) {
         if( contact.thisCollider == collider ) {
                  float english = contact.point.x - transform.position.x;
                 
                 contact.otherCollider.rigidbody.AddForce( 300f * english, 0, 0);
             }
             
         }
     }    
 }