system of particles

...In the case of a ball rolling down a slope, the rotational motion cannot be ignored in addition to the motion of the center of gravity, so it cannot be considered a point mass. Ordinary objects with a certain size are treated as a system of particles made up of a collection of point masses. If we reduce it to microscopic particles such as atoms and electrons, it can certainly be called a system of point masses, but even if we think macroscopically and consider an object as a continuum, if we divide it into tiny parts and consider each of them (which can be considered to contain a large number of atoms) as a point mass, Newton's laws of motion can be applied.

*Some of the terminology that refers to "system of particles" is listed below.

Source | Heibonsha World Encyclopedia 2nd Edition | Information

…斜面を転がり落ちる球のような場合には，重心運動のほかに回転運動を無視できないので質点とはみなせない。大きさをもったふつうの物体は質点が集まった質点系system of particlesとして扱う。原子や電子のような微視的粒子にまで還元すれば確かに質点系といえようが，巨視的に考え物体を連続体とみる立場でも，それを微小部分に分けてそのおのおの(それは多数の原子を含むとみてよい)を質点と考えれば，ニュートンの運動の法則が適用できる。…

※「system of particles」について言及している用語解説の一部を掲載しています。

出典｜株式会社平凡社世界大百科事典 第２版について | 情報