Work Done by Internal Forces on a Multibody System with Constraints

Yukio Kobayashi

Soka University

The work done by internal forces on a multibody system is determined by the constrains which limit the motion of the bodies in the system. At college level, however, some instructors and students do not recognise the distinction between external forces exerted on the bodies of a system and internal forces inherent to the system. Therefore, the work done by internal forces on a system is sometimes confused with that exerted by external forces. Here, the author illustrates by using three examples an approach to the systematic study of the relationship between the constraints imposed on the motion of a multibody system and the work done on the system by internal forces. Even if the absence of external forces causes a constant total momentum in the system, the mechanical energy is not necessarily constant given the work done by internal forces on the system. This difference is explained from temporal and spatial viewpoints of physical phenomena.