The force applied to the rigidbody every frame.

	// Moves the rigidbody upwards in world coordinates
	constantForce.force = Vector3.up * 10;

using UnityEngine;
using System.Collections;

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

import UnityEngine
import System.Collections

public class Example(MonoBehaviour):

	def Example() as void:
		constantForce.force = (Vector3.up * 10)