How does rigidbody works?

I have two block in my scene, block_A and block_B. block_A has both the rigidbody(no gravity) and a box-collider while block_B only has the box-collider. I add a SimpleMove script to the two blocks as below:

 using UnityEngine;
 using System.Collections;
 
 public class SimpleMove : MonoBehaviour {
     public float speed = 10f;
     private Rigidbody body;
     // Use this for initialization
     void Start () {
         body = GetComponent<Rigidbody>();
     }
     
     // Update is called once per frame
     void Update () {
         if (body != null){
             body.AddForce(speed * Time.deltaTime, 0, 0);
         }else{
             transform.Translate(speed * Time.deltaTime, 0, 0);
         }
     }
 }
 

I put the two blocks in a line , and set the speed fields of the script so that they could move forward to each other.

When the speed fields changes, the results are:

1. block_A speed 0 block_B speed 10, block_B will cross block_A.

2. block_A speed -0.1 block_B speed 10, block_B will push block_A to the left.

3. block_A speed -0.1 block_B speed 100, block_B will cross block_A.

So I don't understand these.Thanks if you can help me~!