Collision.relativeVelocity 相对速度

using UnityEngine;
using System.Collections;

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

	}
}

// Play a sound when we hit an object with a big velocity
//当以一个较大的速度碰到物体时，播放声音
function OnCollisionEnter(collision : Collision) {
	if (collision.relativeVelocity.magnitude > 2)
		audio.Play();
}