QUOTE
Slow was initially a mass levitate spell (which is where the 200 kg per hit limit came from). That was obviously pretty damn crazy already, so I limited the effect to a slowing one.
It occurred to me that the slow spell would protect against bullets and shrapnel (I believe it's mentioned in the spell description). I was unconcerned, though, because a lot of things protect against bullets and shrapnel (various barrier spells are really good at it, for example, as are walls). Players have come up with all sorts of clever ways to mitigate those problems, so I had faith in players to handle this one. Players are so ingenious; I recall a story about how an AD&D player overcame an enemy protected by an anti-magic shell by polymorphing a boulder into a small rock and casually tossing it at his target.
Anyway, I've thought of a few ways to get at a target protected by a slow spell. Sorcery is one way, of course. Gas attacks work fine. Spirits with Engulf. Bayonets and many other sharp things that go into soft things that scream and bleed aren't any less dangerous; I tried to make certain that it was clear that forces were not affected, just speed, so you can still shish-kabob someone with a spear or fillet them with a monowhip, it's just more agonizing. Edited to add I'm pretty sure that a compression wave propagates, rather than moves; a physicist can probably tell us how much the air in a location moves during, say, an event caused by a concussion grenade.
I'd love to hear other ways to neutralize the advantage granted by the slow spell. That sort of geeking out is the kind of conversation I dig.
Thanks for the answer.
What if I cast slow on someone wielding a whip? The whip's tip moves at approximating bullet speed. oes the Whip fall down? tear in half because part of it brutally stops and part really wants to move on? Does that go for swords too, whose tips can be awfully fast? what about generators, that have moving partts, does Slow destroy them due to the brutal stop it enforces with one hit? And where is the line drawn between "fast" and "too slow for slow to be effective"? Does Slow affect blood flow? Can slow shut down gyro mounts (spinning parts) and shatter any weapon using one?
Of course, since you want to bring physics in, let's talk about mass:
You said
QUOTE (Aaron in War!)
The spell ends when the caster stops sustaining it or the amount of mass moving in the area of effect exceeds 200 kg per hit on the Spellcasting Test.
Now, mass is usualy equivalent to weight in everyday language. Your usage of the kilogram as a unit certainly affirms this is meant. However (and this may be the venure I will go with fixing this spell) in physics, mass refers to something related, yet different: inertial mass, a concept derived from Newton's second law. Here, an object's mass m is determined by a forece F applied to it and it'a acceleration a.
QUOTE (Newton)
m=F/a
Your spell now decelerates any object to a certain speed: 1 m/s. It does so abruptly. Let's look at the forces at work here:
Say, a bullet with a mass of 10 g moves at 800 m/s and is affected by the Slow spell. Now, if the spell would slow down the bullet instantly, we'd have a force of
infinite power working here, which would break the universe. So let's say the bullet has one combat turn to slow - 3 seconds. Then, we'd be talking about a deceleration of 799 m/s² at work, making for an applicable Force of 2663,[period 3] N (Newton) at work. We get even more force necessary to stop an airdropped soldier.
Now, if we rule that there really wasn't meant to be kg, but N there, then we'd have a spell where stopping bullets will not work any more with just one hit. It'd still work for a soldier being airdropped if we allow it to be played as a sustained spell and an extended test over seveal combat turns (including several instances of drain), which represents using the spell to decelerate more slowly to a minimum of 1 m/s. That way, it is still applicable for what it was supposed to do, but not an instant win to be played against anything that flies, is a kinetic vessel (think about what this spell will do against Rods from God) or has moving parts. Oh, and it won't work well to slow down things that move too fast because it will simply take too long, so no more bullet stop, no more plane crash, and no more blowing up engines with it.
Of course, that fix opens up a whole other can of worms: weight of equipment. Didn't Shadowrun 4 back in the day ditch that for simplicity? But that's a story for another day.