This is a physics topic – maybe not of interest to many, but of interest to me as I happen to be taking (suffering through?) a hydraulics course at the moment. Like energy, we kind of intuitively know what momentum is, but we have a hard time describing it satisfactorily in words. Apparently, there are philosophers of science that spend entire careers examining words used by others (like Isaac Newton) to try to describe it. Once upon a time, a philosopher and a scientist were the same thing, in fact.

Momentum is about force. It is a thing that does not change unless “external forces” are imposed on a system. In fluid mechanics, there is an imaginary thing called a “control volume” which obeys this law. You can do calculations on this, and then you can go into a laboratory where you have a pump and a glass channel (picture a long aquarium) with very low friction, and show that your math matches what happens in the real world. There can be “internal forces” in the fluid which allow energy to change (well, change from energy embodied in pressure – potential energy – and/or velocity – kinetic energy – to heat, which then just drifts off into the air. But the momentum does not change because there are no external forces (ignoring the friction of that slippery, slippery glass.

Momentum is a function of mass and velocity, we learn in high school. Force, we learn in high school, is the product of mass and acceleration, and acceleration is a change in velocity over time. So there – did I explain it to myself? Not quite, but that at least helped me to think it through.

Even if ChatGPT could produce a more coherent version of what I wrote above (which is possible), that would not have helped me think this through and incorporate more of the real world into my mental model of how the real world works. Because thinking and writing go together. So I am not going to give up writing any time soon. Even if nobody read my writing here (and if you did, I apologize), it helped me to write this down. I will skim over this later at some point using some rough version of “spaced repetition”, and that will also help my feeble human brain to incorporate this knowledge into my mental model.

Talking can also sometimes help upgrade our mental models, although most talking is useless. For example, I was discussing momentum with my professor recently in the mens room. So ladies, if you were wondering what men talk about in the mens room, now you know, or at least now you know what two random men were discussing in one random mens room on one random day. And yes, we know you complain about us in the ladies’ room, and that complaining about men is an important part of female bonding that really doesn’t have much to do with us. And this is okay.