title: Allow a rigid body to only move in the Y direction.
body: <p>I am making a 2d game using Unity 4.3. I have a rigidbody2d object. When other physics2d objects hit the rigidbody2d object it moves a little bit in the x direction. </p>
  
  <p>I don't want it to move in the x direction. I want to make the rigidbody2d object's x direction fixed. How can I achieve that?</p>