oncollisionenter not working unity code example

Example 1: unity oncollisionenter2d

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

      	Debug.Log("Collided");
    }

Example 2: unity oncollisionenter

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

      	Debug.Log("Collided");
    }