A hinge attached to one end of a body fixes that end of the body in space. The body may have any orientation in one plane relative to the hinge:
Because of this, a reaction at the hinge will have components both parallel and perpendicular to the surface. We may model the hinge as being rough. The conponent of the reaction force normal to the surface is the normal reaction force, with the ‘roughness’ shown by a tangential component parallel to the surface.
The reaction forceabove may be resolved into vertical and horizontal components for calculations as shown above.