Car made from Different GameObjects and Empty GameObjects breaks on Collision with Terrain.

I made a Car using the following Hierarchy:

I have the following Empty GameObjects(GO): Car, Tyers, Glass, Body, Lights, Head Lights and Rear Lights.

The rest are Basic 3D GameObjects.

There are three Problems: 1) If I have Only the Car GO with the RigidBody then the Car wont move using the RigidBody.AddForce().

2) If I Make Every Empty GO a RigidBody then the Car Breaks and the Empty GOs fall through the Terrain.

3) If I Make Every GO a RigidBody then also the Car Breaks and the Empty GOs fall through the Terrain.

In 2) and 3) the GOs still DON'T respond to RigidBody.AddForce().

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Code Used:

using UnityEngine; using System.Collections;

public class MoveObjectPlayerInputKB : MonoBehaviour {

 private Rigidbody car;

 public float speed = 0.5F;

 public float turnSpeed = 0.5F;

 // Use this for initialization
 void Start () {

     car = GetComponent<Rigidbody> ();
 
 }
 
 // Update is called once per frame
 void Update () {
 
     float turn = Input.GetAxis("Horizontal");
     float forward = Input.GetAxis("Vertical");

     car.AddForce (new Vector3(0F, 0F, forward) * Time.deltaTime * speed);

     transform.Rotate (new Vector3 (0F, turn, 0F) * Time.deltaTime * turnSpeed);

// car.AddTorque (new Vector3 (0F, turn, 0F) Time.deltaTime turnSpeed);

 }

}

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Thanks in advanced and I also Apologize for asking such a Question as I am New to Unity. It's ONLY been 2 weeks.!

The Result I'm getting.