It's good to have handy approximations for D&D purposes. You're about 5' tall. If you stick your arms up, you're 10' tall. A story of a building is around 14', so if you fall off a 10 story building that's 140'. Etc, etc. All GMs eventually come up with their own set of unstated mental yardsticks.
Here are a few handy tips for high-energy physics.
Wizards break rules all the time. Fall off a 10 story building? Hah! Try falling from 40,000' in the air thanks to a botched teleport spell. Use a sphere of annihilation to delete half a portable hole. Loot an ancient nuclear battery and cover it with lead. You know, the usual wizarding things.
We're going to use density a lot, so let's figure out some mental yardsticks.
Humans are approximately as dense as water. In fact, anything that's made of meat or plants is approximately as dense as water. Stone is 3 times as dense as water. Iron is about 8 times as dense as water. Lead (or gold) is about 10 times as dense. We're not going for accuracy, we're going for speed.
So if you fire a tree at a dragon at a bullet-speed? Cake and ripe tomato. Fire a stone cannonball at a stone tower? Cake and ripe tomato. But what happens if you fire an iron dart at a dragon?
This is one of those rules that seems really obvious once it's pointed out.
Momentum must be conserved. m1v1 = m2v2. So if you've got an object of mass m and velocity v heading towards a stationary object, and v is very fast, then all the object can push is the stuff directly in front of it. Once it's moved its own mass "out of the way", it'll stop.
An object's impact depth, in a material of equal density, can therefore be approximated as the object's length.
That's super neat! How deep is the impact crater? About as deep as the object.
How far will bullets go through water? Well, lead is 10 times as dense as water, so a bullet will go only ten times its own length. That's not very far.
And of course this ignores a ton of additional considerations, but it's a very handy rule if you start trying to run through walls or explode bunkers.
3. When In Doubt, Check The '50sAir-to-air. Unguided. Nuclear. Weapons.
Look, the past is a different country, but some parts of that country are ludicrously dangerous. I'm sure very sensible papers and sober papers were written at the time on the absolute necessity of unguided air-to-air nuclear weapons... but... really?
Anyway, people (mostly the incomparable Alex Wellerstein) have created tools for visualizing various nuclear scenarios. Playing around with Nukemap will give you a few mental yardsticks for any potential nuclear disasters in your RPGs.
Fallout-style nuclear cars, alien power sources, ghost-busting rayguns, or other high-tech but portable devices can be approximated as the adorably murderous Davy Crockett portable nuclear device. Should said device detonate in the hand of a hapless PC, One large building disappears, a city block is leveled, around 3 blocks are scorched and shattered, and 9 blocks are irradiated to the point of medical consequences.
4. At Sufficiently High Velocities, Everything Behaves Like A Fluid"Fluid" is a bit like "fish", in that it makes some degree of intuitive sense but falls apart when examined. Still, if you're dealing with velocities faster than the eye can follow, using your intuition towards pudding is probably not a bad idea.
5. You Don't Have Time To Un-Fuck UpHigh-energy phyiscs, in general, doesn't deal with round-by-round D&D very well. Round 1, you're a sauropod eating plants in a shallow swamp. Round 1.000001, you and everything you've ever seen is plasma. Fast extinction-type asteroids don't go skittering across the sky like meteors. They go from space to ground level between refresh cycles on your monitor. There's less of a shockwave and more of a shockwall. It's a nice sunny day until everything suddenly goes white.
Take some radioactive material from subcritical to critical? Your reflexes aren't faster than a speeding neutron. Decide to rewire your bathroom while the wires are live? Your reflexes aren't faster than AC. Etc.
The OSR generally supports the notion that you can make one really bad choice, then a whole string of good ones, and still be utterly fucked over when that bad choice hits maturity. So does high-energy physics.