"The gravitational mass - characteristic of a material point in the analysis of classical mechanics, which is assumed to cause the gravitational interaction of bodies, in contrast to inertial mass, which determines the dynamic properties of bodies."
In this definition, the definition of inertial mass does not grasp its essence. Gravitational mass can be at rest or in motion. Therefore, we say that there is the rest mass and mass inertia. The kinetic energy of the rest mass is equal to zero, and therefore its (rest mass) speed is equal to zero, i.e., it is in a relative state of rest. In contrast inertia mass has a kinetic energy that it receives as a result of interaction with another body, so it has a speed corresponding to this energy. But if we take away this kinetic energy little by little in this inertial mass (for example, by using friction), the body will slow down its movement and stop when its kinetic energy will be equal to 0, i.e., inertial mass becomes a rest mass. But in nature there are such phenomena when under the motion of the body, its mass gets kinetic energy and at the same time it gives this energy away. In this case we have to deal with the gravitational rest mass of the moving body. Below I will highlight one of these phenomena.
The existence of the charged and uncharged body mass determines its different behavior during the interaction, on the one hand, this body with another body, but on the other hand, the same body with charge in the electrostatic field of another charge. To prove this, we consider the motion of uncharged and charged body under the action of mechanical force and motion of the same body, but the mass of which has a charge, in an electrostatic field.

Example 1. Assume that we have a body with uncharged mass M, which lies on the plane at point A, and the initial velocity and kinetic energy of which is equal to 0. For simplicity, we neglect the friction. Now, the mechanical force F, directed horizontally, applied to this body. In this case, our body begins to move with constant acceleration if the force does not change, but if it changes, then the body will move with increasing acceleration (second Newton's law). If at some distance from point A let's say at point B , we remove the force F, then according to Newton's First Law the body will continue to move but in uniform motion with the same speed. The energy that we passed to the body from point A to B, turned into kinetic energy of the body. And due to this energy, the body continues its uniform motion. We can also say that the mass of our body is inertial because it moves by inertia.
From the standpoint of energy conservation. We have one source of mechanical energy, which is entirely converted into kinetic energy of the moving body. These arguments are well known and no one in doubt.

Example 2. Let's take the same conditions as in Example 1, but our body will be charged with charge q, and consider whether the behavior of the mass of our body in this case will be changed or not. In other words, firstly, whether the mass of our body will maintain inertial properties or not and, secondly, what new happens in this process. Since the mass of our body moves due to interaction with the mass of another body, i.e., mechanical force F, the energy expended on the motion of our body, as before, will be maintained by the mass of the moving body, i.e., our body at point B will have a kinetic energy M VV / 2, so that body mass retains its inertial properties.
But as we deal with the accelerated movement of our charged body, then this charge will generate electromagnetic waves, for the simple reason that our body, in this case, moves in both the gravitational and the electrostatic field of the Earth. The formation of this electromagnetic energy does not consume a single drop of kinetic energy of our moving body, because as I said earlier in a previous article ("One-sided understanding of the concept of" relative energy "), the mechanical energy is transferred only to the mass of our moving body and not to its charge.
From the standpoint of energy conservation. Here we have two power sources: a source of mechanical energy, which is entirely converted into kinetic energy of the moving body, let this energy call E1 - and a source of electrical energy, which is the electrostatic field of the Earth. It is Energy of the field is spent on electromagnetic radiation, which is produced by our charge q under mechanical motion of the body. We denote this spent energy of the field as E2. The question arises: what the energy E2 is equal to. The answer to this question is very simple. It is equal to the kinetic energy of our moving body, i.e., E2 = E1. This equality is evident when considering Example 3.

Example 3. Let's take the same body, with mass M and with charge q, and force it to move on the same line AB, in the area of a strong electrostatic field of charge Q, due to the force F1 which is the result of interaction of the charges, q and Q. In our case F1 will always be equal to the mechanical force F. Then, according to the second law of Newton, qE = ma, our body will move along line AB with the same acceleration as in the example 2 and 1. Next, at point B, we also interrupt the action of this force by neutralizing the charge q to a minimum but >0.

The question arises: Is the first law of Newton valid, in this case?

In other words, whether the body would continue its movement by inertia with the same speed as in the example 1 and 2, or it will stop right away.

My answer to this question is negative, i.e., the body immediately stops and here's why. If in Example 2, with the mechanical motion of a charged body, the energy stored in the mass of the charged body, then under the motion of a charged body due to electrostatic forces of the field the situation is reversed. The energy of the Electrostatic field is not conserved by the mass of our body but transferred to the charge q due to which the electromagnetic waves are created, which I reviewed in Example 2. In other words, the mechanical energy is transferred only to the mass of the body but not to its electric charge, and electrostatic energy is transferred only to the electrical charge of the body rather than to its mass. For this reason, if you stop the effects of the electrostatic force on a charged body, then at that point body mass will not have any kinetic energy, because all the energy went into the formation of electromagnetic waves by the accelerated motion of the charged body. If so then our body after disabling impact force F1 will not have any kinetic energy, which is the source of inertial motion. Therefore, our body stops.
From the standpoint of energy conservation. In this example 3, as I said, the charged body , moving with the same acceleration, under the influence of the electric force F1, as in Example 2 under the force F. Consequently, on the motion of the charged body is spent in both cases the same quantity of energy but different by its nature of creation. And because all this energy, in Example 3, is not accumulated in the mass of the body and completely transmitted to the charge q, which generates an electromagnetic wave, it follows that the mechanical energy E1 is equal to the electromagnetic energy E2 in Example 2.

                                                                                             CONCLUSION
My substantiation that the gravitational charged mass of the body, during its motion in the electrostatic field due to its energy, does not behave like inertial mass but as the rest mass - leads us to the establishment of a new law about the motion of a charged body under these conditions.

In the electrostatic field, a charged body retains its movement in general until the force of interaction between charges is applied.
This new law is completely opposite to the first law of Newton, which is valid only for the motion of bodies under the force of interaction of bodies like the masses. But if a charged body moves in an electrostatic field under the action of its forces, then Newton's first law is invalid in this case, and gives way to our new discovered law. It is in this sense that I distinguish the behavior of the body, on the one hand, as the body-mass, and on the other hand, as the body-field.