Because internal forces are capable of changing the form of energy without changing the total amount of mechanical energy, they are sometimes referred to as conservative forces. In the following descriptions, the only forces doing work upon the objects are internal forces – gravitational and spring forces.

Internal forces are forces exchanged by the objects in the system. Internal forces may cause acceleration in different parts of the system but does not cause any acceleration in the center of mass of the entire system. Example: Friction is an external force if the body experiencing friction in the system.

There three types of internal forces: the axial force, also known as the normal force, or a compression or tension force, acting aligned with the extension of a structure member; shear force, a force acting in a direction perpendicular to the alignment of the member; and moment force, a turning result of a force …

The tension is not an internal force for [pulley+block] system. Simply because the string is not a part of the system. However, if you consider [pulley +block +string] as a single system, then of course, the tension is an internal force because it is being applied by objects in the system (string).

Notice about the work by internal forces: the work by the internal forces is in general not equal to zero although the sum of the internal forces is always equal to zero. the net work done by internal forces is zero since they do not cause motion of the object.

The internal forces are very important: Their magnitudes and directions are the deciding factors in the required size and form of the structural components.

Work of Internal Forces Since internal forces always have equal magnitudes and opposite directions, the work done by one internal force cancels the work done by its equal and opposite internal force. In other words, the resultant work done by the pair of internal forces is zero in the case of rigid body.

To calculate these internal forces, simply:

1. Draw a free-body diagram of the entire body,
2. Find reactions at external supports,
3. Find reactions at connections,
4. Keep all loads in their exact locations,
5. Pass a section cut through the member perpendicular to its axis at the point where the internal loads are to be determined,

1)Internal forces can never ever change the total mechanical energy(KE+PE) of an object, but they can just convert the energy from one form to another. This means they can convert a part of KE to PE, which implies they can change the KE but the total (KE+PE) remains conserved.

Internal forces are the forces that emanate from within a system. The system must be defined by you/problem. External forces are the forces that emanate from outside the system. Action or reaction force has no difference.

External forces are forces resulting from the interaction between human body and its environment. External forces can be divided into contact forces and non-contact forces. Most forces that biomechanics works with are contact forces. Gravitational force, acting on all objects on the Earth, is a non-contact force.

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.

The five types of loads that can act on a structure are tension, compression, shear, bending and torsion.

Compression Force is the application of power, pressure, or exertion against an object that causes it to become squeezed, squashed, or compacted. Objects routinely subjected to compression forces include columns, gaskets, disc brakes, and the components of fuel cells.