The mass of the body can be judged by its weight. The greater the mass of the body, the greater the weight of this body,
This property of bodies makes it possible to compare by weight not only the masses of bodies from the same substance, but also the masses of bodies from different substances. Very precise experiments have established that if the weight of a body from one substance is five times, for example, more than the weight of another body from another substance, then the mass of the first body will be five times more than the mass of the second body.
When we compare the weights of bodies with the weights, we also compare the masses of these bodies."
As you can see from this quote, the article on mass in the sixth grade does not even mention the concept of "inertia", and the main emphasis is on the fact that mass is defined through weight. This of course greatly confused the children, and they had the illusion that mass and weight is the same thing. Moreover, at that time the unit of mass was a kilogram, abbreviated as kg, and the unit of weight was expressed in kilograms, abbreviated as kG. Here is a quote from the textbook: "A unit of mass is called, like a unit of weight, a kilogram, but unlike a unit of weight (1 kG), it is designated 1 kg." The same name for the units of mass and weight caused even more confusion in children's minds, and only when they studied Newton's second law did they begin to understand the difference between weight and mass (and even then, perhaps, not all), since the unit of weight is numerically not equal to the unit of mass. However, this already applies to the methodology of teaching physics in high school. Here we draw the readers ' attention to the fact that the teachers knew, of course, that mass, in addition to being a quantity of matter, has inertial properties. They knew that mass is a measure of the inertia of a body. So, in 1960, two things were noted about mass:
1. Mass is a measure of the inertia of a body.
2. Mass is the amount of a substance.
Newton was more precise on the second point: 3. Mass is the amount of matter.
Actually, the amount of something is determined by the count: one, two, three, … ten, …thousand, million, … billion, … and so on. Newton, of course, did not understand this concept of "quantity" in this way, but he understood it closely to this concept. How exactly? As described in the textbook for the sixth grade: the mass of a loaf of bread is less than the mass of a loaf, or the mass of a stone is more than a piece that was taken from it. In other words, Newton declared everything contained in the body to be the amount of matter. In a large piece – the amount of matter is much, in a small piece-the amount of matter is small. But since mass is also a measure of inertia, a large amount of matter has a large inertia, which means that it is more difficult to bring it out of a state of relative rest or uniform rectilinear motion compared to a small amount of matter. Or, in other words, with the same effect on a large and small amount of matter, the result will be different. The result for our case is acceleration. Therefore, with the same force on a large and small amount of matter, the acceleration of a large amount of matter will be small, and the acceleration of a small amount of matter will be large, which, in fact, follows from Newton's second law.