Collision.relativeVelocity

Description

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

// Play a sound when we hit an object with a big velocity
using UnityEngine;
using System.Collections;

public class ExampleClass : MonoBehaviour
{
    AudioSource audioSource;

    void Start()
    {
        audioSource = GetComponent<AudioSource>();
    }

    void OnCollisionEnter(Collision collision)
    {
        if (collision.relativeVelocity.magnitude > 2)
            audioSource.Play();
    }
}