As is the case with the third law, in physics we often have to simplify in order to make measurement and observation possible. We used to make these simplifications to a system of two interacting bodies, and we ignored any other external forces affecting the system. More precisely, with this action, we defined an isolated and ideal system, which was formed solely of two interacting bodies, and the world of our physical observations was limited to these two objects and their interactions.
In fact, such simplifications guided us to understand some of the most important principles governing the natural phenomena, that is, the laws of conservation. We investigate three conservation laws that are of fundamental importance to us:
First, the law of “conservation of mass”, states that the mass in a closed system always remains constant. The law implies that mass can neither be created nor destroyed.
Second, the law of “conservation of momentum”, states that in an isolated and ideal system, including any number of interacting objects, the momentum of the whole system remains constant. Therefore, if the external forces affecting an assumed system are zero, the momentum of the whole system will not change even if the momentum of the internal particles of the system change.
Third, the law of “conservation of energy”, states that the amount of energy in an isolated and ideal system, which itself is a combination of the kinetic and the potential energy of the particles of the system, always remains constant. The consequence of this rule is that energy can neither be created nor destroyed; rather, it can only be transformed from one form to another.
The laws of conservation are due to the unique features of our world and are valid only under certain conditions. These rules have always been somehow involved in discovering new physics findings or any physical observation and experimentation.