Internal external forces:

If 2 parts of a system exert forces on each other, those forces are said to be internal forces for system. An external force is a force among a part of a system and outside the system. So doesn't whether a force is internal or external depend on what you define as your system.

There are a number of ways to categorize all types of forces. It was mentioned that all types of forces can be categorized as contact forces or as action-at-a-distance forces. Whether a force was categorized as an action-at-a-distance force was dependent upon whether or not that type of force could be their even when the objects were not touching physically. The force of gravity, magnetic forces and electrical forces were examples of forces which could prevail between 2 objects even when they are not touching physically. In this lesson, we will learn to categorize the forces based upon whether or not their presence is capable of changing the object's total mechanical energy. We will learn that there are a number of types of forces, which when present and when involved in doing work on objects will change the whole mechanical energy of the object. And there are other types of forces which can never change the whole mechanical energy of an object, but rather can transform the energy of an object from potential energy to kinetic energy. The 2 categories of forces are referred to as external forces and internal forces.

Forces can be categorized as external forces or internal forces. There are many sophisticated and worthy ways of explaining and distinguishing between external and internal forces. Many of these ways are discussed at great length in physics textbooks. For our purposes, we will simply say that external forces include the applied force, tension force, normal force, friction force, and air resistance force. And for our purposes, the internal forces include the magnetic force, gravity forces, electrical force, and spring force.