I am inspired by physics today.
I think I have newton's laws correct. They might be backwards.
An animated visualization of planets orbiting.

Actually, it seems the planets are repelling. Oops.
This is a good exercise because it has already run into a quantization bug.

The planets fall into each other, rapidly increase toward infinite gravity,
and slingshot themselves off and away, and the timesteps are too large to see the process.
Most of the commit is debugging cruft. I haven't solved it yet.