A system of material points internal and external forces

7 A system of material points internal and external forces 

Any set of MPs (or bodies) is called a material points system. Each system s points can interact both with bodies of the same system and with bodies not belonging to it. Forces acting between the system s MP (bodies) are referred to as internal forces. Forces acting on the system s points from ontside are referred to as external forces. A system is called closed (or isolated) if it comprises all interacting bodies. Thus, in the closed system only internal forces are acting. They compensate each other according to the third Newtonian law. 

Strictly speaking, there are no closed systems in nature. However, it is almost always possible to define a task neglecting external forces in the limit of accuracy. The choice of the border surface is a person s own prerogative and can be chosen by the researcher on the basis of an analysis of internal and external forces. One and the same system can be considered as closed or open in different situation, depending on the statement of the problem and pre-given accuracy. All processes are described more easily and decided more clearly in the closed system. This is what in physics is called the physical model. 

In many cases, analysis of a problem s symmetry simplifies its solution. We use this in many sections of this book. 

Let us introduce a very important notion, a centre of mass (CM) (or the center of inertia). The CM of a system is the point at which the system s mass can be assumed to be concentrated. It can be described with a radius vector 

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