What are internal and external forces?
External forces
External forces are forces caused by external agent outside of the system. Internal forces are forces exchanged by the objects in the system.
External forces are forces applied to the structure from the outside of the structure. Examples of external forces include dead loads, such as the weight of the structure itself and the non-structural materials it supports, and live loads, which include moving loads, such as occupants, goods, and furniture, as well as wind loads, seismic loads, and impact loads, among others.
A reaction force at points where the structural member is supported resulting from external forces is also an external force. In addition, despite the lack of connection between two or more objects, forces such as gravitational and electromagnetic forces can also be classified as external forces.
Internal forces
Any force that acts on a structure from within is known as the internal force. An internal force resists the motion. Internal forces are produced from the external forces acting on structure members such as pole, beam, or column. Generally, we have three types of internal forces: Axial, Shear, and Moment forces.
Axial force
Axial force sometimes called 'normal force,' is a compression or tension force acting aligned with the extension of a structure member.
Shear force
Shear force is a force acting in a direction perpendicular to the alignment of the member.
Moment force
Moment force is a turning result of a force multiplied by the distance from its acting location to the turning point.