On collision enter not being activated

I am trying to create pong with unity 4.3 as a beginner learning project. As part of this I would like the ball to have a little bit of randomness apon collison, in order to do this i have the following in the ball script:

 void OnCollisionEnter(Collision c) {
         Debug.Log("afjhas");
     }

However it appears that the function is never being activated upon the ball colliding with another object. The objects that they need to collide with possess a rigibody 2d and a 2d colliding box.

The full script of the ball is:

 using UnityEngine;
 using System.Collections;
 
 public class Ball_scr : MonoBehaviour {
 
     // Use this for initialization
     void Start () {
     Spawn ();
     }
     void Spawn(){
         Debug.Log("dasdaf");
 
         transform.position = new Vector3(0.0f,0.0f, transform.position.z);
         Vector3 newpos=gameObject.transform.position;
 
 
         System.Random rand=new System.Random();
         if(rand.NextDouble() > 0.5){
 
             newpos.x =5;
         }else{
             newpos.x =-5;
         }
 
         newpos.y=(float)GetRandomNumber(-2,2);
         gameObject.rigidbody2D.velocity=newpos;
     }
     double GetRandomNumber(double minimum, double maximum)
     { 
         System.Random rand=new System.Random();
         return rand.NextDouble() * (maximum - minimum) + minimum;
     }
 
     void OnCollisionEnter(Collision c) {
         Debug.Log("afjhas");
     }
 }