The entire model of sea-borne power projection has changed. What does it mean when hypersonic missiles can be fitted with optical sensors to autonomously target ships? It means carriers now are about as useful as battleships in WW2. Nobody knows who can dominate which parts of the world ocean anymore.
Several years ago I ran into this project [0] and got overwhelmed even the algorithm can be written in 88 lines of C++. I realized that out of all CS topics, physical simulation is probably the one I knew the less (not saying I'm a compiler/database expert or something, but at least I've implemented a toy compiler and some basic data structures used in database. When it comes to physical simulation my bran just draws a blank.)
[0]: https://github.com/yuanming-hu/taichi_mpm
I'll give you a simple example. For diffusion of heat between 2 points, the rate of change (first derivative) is proportional to the difference in temp between them. So you make an update rule for points on a grid that says "calc the average difference of a cell's temp with that of its neighbors, multiply by some constant, and that is the amount to update this cell at this time. Run that for every cell in parallel, many times." Then you tack on a visualization and you can watch the heat diffuse. A fun example would be the cooling of the proto-Earth. You can watch the crust form.
Heat diffusion is a good starter problem. So is gravitational interaction.