Even in the school curriculum, there is a provision that any movement of one body can be fixed only relative to another body. This position is called the term "relativity of motion." From the pictures of the textbooks, it was clear that for a person standing on the river bank, the speed of a boat sailing past is the sum of its speed and the speed of the river. After such a detailed examination, it becomes clear that the relativity of movement surrounds us in all aspects of our lives. The speed of the object is a relative value, but its derivative, acceleration, also becomes a relative value. The importance of this conclusion is that it is acceleration that is included in the formula of Newton’s second law (the basic law of mechanics). According to this law, any force acting on the body gives it an acceleration proportional to it. The relativity of movement makes us ask an additional question: regarding which body is acceleration given?
There are no explanations in this law on this subject, but by simple logical conclusions, we can conclude that, since force is a measure of the influence of one body (1) on another (2), this same force gives the body (2) acceleration relative to the body (1), and not just some abstract acceleration.
Relativity of movement is the dependence of a certain trajectory of a body, a certain path, speed and movement on selected reference systems. In the aspect of kinematics, any applied reference systems are equal, but at the same time, all the kinematic characteristics of this movement (trajectory, speed, movement) are different in them. All quantities that depend on the selected reference frame with which to measure them are called relative.
The relativity of motion, the definition of which is quite difficult to give without a detailed examination of other concepts, requires accurate mathematical calculation. It is possible to talk about whether the body moves or not, when it is absolutely clear about what (the reference body) its position changes. The reference system is a combination of such elements as the reference body, as well as the associated coordinate systems and time reference systems. In relation to these elements, the movement of any bodies or material points is also considered. Mathematically, the movement of an object (point) with respect to the chosen reference frame is described by equations that establish how the coordinates that determine the position of the object in this system change over time. Such equations determining the relativity of motion are called equations of motion.
In modern mechanics, any movement of an object is relative, therefore it should be considered only in relation to another object (reference body) or an entire system of bodies. For example, one cannot simply indicate that the moon is moving at all. The correct statement will be that the moon moves in relation to the sun, earth, stars.
Often in mechanics and the theory of relativity, the reference system is not linked to the body, but to a whole continuum of basic bodies (real or fictional) that define the coordinate system.
In movies, they often show movement relative to various bodies. So, for example, in some frames they show a train that moves against the background of some landscape (this is movement relative to the surface of the Earth), and in the next frames - a compartment of a car, in the windows of which trees flicker (movement relative to one car). Any movement or rest of the body, which is a special case of movement, is relative. Therefore, answering a simple question, does the body move or rest, and how does it move, it is necessary to clarify with respect to which objects its movement is considered. The choice of reference systems, as a rule, is made depending on the set task conditions.