“Body mass is a physical quantity that is a measure of the inertia of a body.”
Thus, mass became a calculated physical quantity and turned from the basic physical units into a derivative of speed. This in no way overcame the paradox we are considering, but on the contrary, only aggravated it. As you know, the speed is relative, if you do not take into account the speed of light, but photons do not have mass, so the speed of light, we are not interested here. Due to the relativity of the nature of speed, it turned out that the same piece of stone has a different mass depending on the accepted reference system. Thus, the last paradox was worse than the first. Einstein's epigones have only to confuse and blab the definition of mass, so that it completely loses any meaning. Let's see how our modern Wikipedia now interprets the definition of "mass".
"Mass" is a scalar physical quantity that determines the inertial and gravitational properties of bodies in situations where their speed is much less than the speed of light."
Oh, how simple, the reader will be happy – in this definition, the inertial properties of mass were only added to the gravitational properties and that's all. Don't be happy, reader. First, because we got rid of the gravitational properties of the mass of substance in the first issue of this series. Substance to substance is not attracted, and even more so mass to mass. The mass is attracted, but not to the mass, but to the light (positive) matter, which just has no mass. [1]. Secondly, the main thing for the conformists is to get rid of the mass, as the amount of matter, or the amount of matter, then you can do with it whatever your heart desires. In addition, the conformists left this definition of mass in science for engineers, schoolchildren, and ordinary people. For "smart" people, they will offer the most rigorous definition of mass.
We will continue to quote Wikipedia: "Being closely related to such concepts of mechanics as "energy" and "momentum", mass manifests itself in nature in two qualitatively different ways, which gives grounds for dividing it into two varieties:
The inert mass characterizes the inertia of bodies and appears in the expression of Newton's second law: if a given force in the inertial frame of reference accelerates different bodies equally, they are assigned the same inert mass;