The equation for net external force is the sum of all the external forces applied to the object. For example, a compilation of common external forces is friction force, normal force, air resistance, tension and applied force. Because these are vectors, adding them together produces the correct total external force.
Although there are also internal forces acting on a body, these do not contribute to the total mechanical energy of a body when work is done on it, which is the quantity of greatest interest in calculations of external force. Work is the application of external force on a body to move it a certain distance.