Starting to move, the body tends to continue moving. The first law of Newton’s mechanics states: if the body moves, then in the absence of external influences it will continue to move straight and evenly until it is subjected to an external force. This tendency is called a linear impulse. We often encounter it in everyday life. The billiard ball rolls on the table with the speed that is given to it by the cue, the spear flies with the speed with which it was thrown.
Physicists determine the linear momentum of the body p as its mass m multiplied by its velocity v :
p = mv
The letters p and v are bold in order to show that these quantities are characterized not only by the absolute value but also by the direction. So, with respect to speed, we do not just say that the car is moving at a speed of 40 km / h and that it is moving at a speed of 40 km / h, for example, to the north. A quantity that has a direction other than an absolute value is called a vector.
It is clear that, according to Newton’s first law, the amount of motion of a single body in the absence of external forces is preserved. The law of conservation of momentum says that if this condition is met, the vector sum of the momenta of all bodies entering the closed mechanical system is preserved. In such a representation, a system of two billiard balls of mass m launched opposite to each other with identical velocities v, will have a zero angular momentum, although each of the balls separately has a momentum mv. However, the impulses of the balls mutually cancel out due to their vector nature (since their velocities are oppositely directed).
In general, any quantity that characterizes the system and does not change as a result of interaction within it is called conservative, and for it, there is a conservation law. In particular, in mechanical systems, in addition to the law of conservation of momentum, the law of conservation of the angular momentum or quantity of rotation also acts, a quantity that describes the amount of motion of bodies around its own axis and along curved trajectories.
What happens with a rectilinear collision of two billiard balls in a collision course? There are several phenomena at once. First, at the moment of the collision, the spheres are slightly deformed and part of their kinetic energy passes into the thermal one. Secondly, we know that the total momentum of a system of two balls does not change and remains zero. Hence, seeing that one ball rolls back after a frontal collision in the opposite direction at a certain speed, we can say with certainty that the second ball at the given moment of time rolls in the opposite direction with exactly the same speed.
The second law of Newtonian mechanics, by the way, can easily be interpreted as a formula according to which the rate of change of momentum is equal to the force applied to a closed system. Thus, to change the momentum of the system, an external force is required. In the molecular-kinetic theory, for example, this is clearly seen: the pressure is explained by the momenta of the impacts of molecules on the wall of the vessel containing the gas. Since the molecules of the gas spring back in an elastic manner, their impulses change to opposite directions, which means that the wall exerts a force on the molecules striking about it. But this means that the molecules, by virtue of Newton’s third law, exert a force on the wall, which is perceived by us as pressure