When summing forces in the x direction why does Force of Ax be included? Wouldn\'t Ax not be included because it is considered an internal force? Solution Ax is an external force that\'s why it is included in calculating the net force in a direction .Internal forces have no role in FBD of a body as the are cancelled on its on by action reaction. Ax is included in the net force in the X-direction because it is one of the force that is applied externally and bring a change to the system..