Check out example codes for "unity oncollisionenter". It will help you in understanding the concepts better.

Code Example 1

void OnCollisionEnter(Collision col)
    {
		//This method will run when your game object
      	//collides with something

      	Debug.Log("Collided");
    }

Code Example 2

using UnityEngine;
using System.Collections;public class ExampleClass : MonoBehaviour
{
    AudioSource audioSource;    void Start()
    {
        audioSource = GetComponent<AudioSource>();
    }    void OnCollisionEnter(Collision collision)
    {
        foreach (ContactPoint contact in collision.contacts)
        {
            Debug.DrawRay(contact.point, contact.normal, Color.white);
        }
        if (collision.relativeVelocity.magnitude > 2)
            audioSource.Play();
    }
}

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