When trying to write the equations to describe the mechanics of atoms, it was discovered that the classical mechanics of Newton wouldn’t work. This is because the pieces of an atom, the nucleus, electrons, etc, are quite small, and on that scale, there are qualities of how physical bodies behave which we don’t notice in everyday life, because everything we are used to seeing is much bigger.
If you take a class on quantum mechanics, there are a handful of problems that you end up considering over and over. One is the harmonic oscillator, in which a body is attached to a spring and oscillating back and forth (the other end of the spring is hooked to something bigger that doesn’t move). Another is the rigid rotor, which is two bodies connected at a fixed distance, like a dumbbell, and spinning.
These problems are ways of modeling atoms and their parts. The harmonic oscillator could be an electron wiggling back and forth in relation to the nucleus. The rigid rotor could be a hydrogen molecule, which is two hydrogen atoms bound together.
