Check out example codes for "unity oncollisionenter2d". It will help you in understanding the concepts better.

Code Example 1

void OnCollisionEnter2D(Collision2D col)
		//This method will run when your 2D game object
      	//collides with something


Code Example 2

using UnityEngine;
using System.Collections;public class ExampleClass : MonoBehaviour {
    void OnCollisionEnter2D(Collision2D coll) {
        if (coll.gameObject.tag == "Enemy")
            coll.gameObject.SendMessage("ApplyDamage", 10);

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