The force - relative to the rigid bodies coordinate system - applied every frame.

	// Moves the rigidbody forward
	constantForce.relativeForce = Vector3.forward * 10;

using UnityEngine;
using System.Collections;

public class Example : MonoBehaviour {
    void Example() {
        constantForce.relativeForce = Vector3.forward * 10;
    }
}

import UnityEngine
import System.Collections

public class Example(MonoBehaviour):

	def Example() as void:
		constantForce.relativeForce = (Vector3.forward * 10)