Physics in 2D mode not working?

I have a ball that I want to simulate with 2D physics to bounce around when dropped, but I have a problem. If i just add a rigidbody2D to the sprite it just starts to float up. If I try to add any force using rigidbody2D.AddForce() it moves fast for one frame then starts to slowly drift upwards again. Yes i have checked the Y-axis and it is pointing up. Here is the code for adding force.

 if(Input.GetMouseButtonDown(0))
 {
     Vector2 force = ((Vector2.right + Vector2.up).normalized * m_force_thrown);
     Debug.Log(force);
     transform.rigidbody2D.AddForce(force);
 }

Any Idea what is going on, or how I can make it act naturally?