Relativity of motion

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 the person standing on the bank of the river , the speed of the boat floating past is composed of its speed and the speed of the current of the river. After such a detailed analysis it becomes clear that the relativity of the movement surrounds us in all aspects of our life. The speed of the object is a relative value, but the derivative of it, the acceleration, also becomes a relative value. The importance of this conclusion is that it is acceleration that is part of the formula of Newton's second law (the basic law of mechanics). According to this law, any force acting on the body gives it a proportional acceleration. The relativity of motion makes one ask an additional question: what kind of body is accelerated?

In this law, there is no explanation in this regard, but by simple logical inferences it can be concluded that since force is a measure of the impact of one body (1) on another (2), this same force tells the body (2) the acceleration relative to the body (1), and not just some kind of abstract acceleration.

Relativity of motion is the dependence of a certain trajectory of motion of some body, a certain path, speed and displacement from the selected reference frames. In the aspect of kinematics, any reference frames used are equal, but all the kinematic characteristics of this motion (trajectory, velocity, displacement) are different in them. All quantities that depend on the selected frame of reference, with which they will be measured, are called relative.

Relativity of motion, the definition of which is rather difficult to give without a detailed consideration of other concepts, requires an accurate mathematical calculation. Talk about whether the body moves or not, it is possible when it is absolutely clear as to what (the reference body) its position changes. The reference system is a collection of such elements as the reference body, as well as the coordinate systems and the time reference system associated with it. In relation to these elements, the motion of any bodies or material points is considered. Mathematically, the movement of an object (point) with respect to the selected frame of reference is described by equations that establish how the coordinates change in time, which determine the position of the object in this system. Such equations, which determine the relativity of motion, are called the equations of motion.

In modern mechanics, any movement of an object is relative, so it should be considered only in relation to another object (the reference body) or an entire system of bodies. For example, you can not just indicate that the Moon is moving at all. The correct statement is that the Moon moves in relation to the Sun, Earth, stars.

Often in mechanics and the theory of relativity the frame of reference is not linked to the body, but to the whole continuum of base bodies (real or imaginary) that define the coordinate system.

In motion pictures quite often show movement concerning various bodies. For example, in some frames a train is shown that is moving against a background of some landscape (this movement is relative to the surface of the Earth), and in the next - a compartment of the car, in the windows of which the trees flash (motion relative to one car). Any movement or rest of the body, which is a particular case of motion, is relative. Therefore, answering a simple question, the body moves or is at rest, and how it moves, it is necessary to specify which objects are considered its motion. The choice of frames of reference, as a rule, is made depending on the set conditions of the problem.