The relative linear velocity of the two colliding objects (Read Only).

	// Play a sound when we hit an object with a big velocity
	function OnCollisionEnter(collision : Collision) {
		if (collision.relativeVelocity.magnitude > 2)
			audio.Play();
	}

using UnityEngine;
using System.Collections;

public class Example : MonoBehaviour {
    void OnCollisionEnter(Collision collision) {
        if (collision.relativeVelocity.magnitude > 2)
            audio.Play();
        
    }
}

import UnityEngine
import System.Collections

public class Example(MonoBehaviour):

	def OnCollisionEnter(collision as Collision) as void:
		if collision.relativeVelocity.magnitude > 2:
			audio.Play()