Scripting Reference
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Rigidbody.AddRelativeForce

function AddRelativeForce (force : Vector3, mode : ForceMode = ForceMode.Force) : void

Description

Adds a force to the rigidbody relative to its coordinate system.

As a result the rigidbody will start moving.

JavaScript 
    // Moves the rigidbody forward along its own z-axis
    function FixedUpdate () {
        rigidbody.AddRelativeForce (Vector3.forward * 10);
    }
using UnityEngine;
using System.Collections;

public class example : MonoBehaviour {
    void FixedUpdate() {
        rigidbody.AddRelativeForce(Vector3.forward * 10);
    }
}
import UnityEngine
import System.Collections

class example(MonoBehaviour):

    def FixedUpdate():
        rigidbody.AddRelativeForce((Vector3.forward * 10))

function AddRelativeForce (x : float, y : float, z : float, mode : ForceMode = ForceMode.Force) : void

Description

Adds a force to the rigidbody relative to its coordinate system.

As a result the rigidbody will start moving.

JavaScript 
    // Moves the rigidbody forward along its own z-axis
    function FixedUpdate () {
        rigidbody.AddRelativeForce (0, 0, 10);
    }
using UnityEngine;
using System.Collections;

public class example : MonoBehaviour {
    void FixedUpdate() {
        rigidbody.AddRelativeForce(0, 0, 10);
    }
}
import UnityEngine
import System.Collections

class example(MonoBehaviour):

    def FixedUpdate():
        rigidbody.AddRelativeForce(0, 0, 10)
If you want to apply a force over several frames you should apply it inside FixedUpdate instead of Update.