Unity2D game objects don't collide

I made two objects and attached a BoxCollider and a RigidBody2D component to each of them.

In the code associated with object 1, I have

     void FixedUpdate(){
         GetComponent<Rigidbody2D> ().AddForce (new Vector2 (1.0f, 9.8f),
                                                   ForceMode2D.Force);
     }
     void OnCollisionEnter(){
         Debug.Log ("Collision");
     }

and in the code associated with object 2, I have,

     void FixedUpdate(){
         GetComponent<Rigidbody2D> ().AddForce (new Vector2 (-1.0f, 9.8f),
                                                    ForceMode2D.Force);
     }

And when I play, the objects move toward each other, seem to physically touch each other, and then rotate away from each other. Nothing is logged, OnCollisionEnter is never entered into?