[RPM] Potential Energy
 

... or, how to make falling boulders and boxes of explosives in RPM.

This thread is inspired by this ritual and the problem it raises. I think I actually have a relatively simple solution.

See, the issue isn't so much creating a big boulder that can fall on and smash people - it's creating one up in the sky above the character. After all, if someone were to create a boulder right beside them, then push it off a cliff, that doesn't seem an illegitimate option (it's just like being able to create a club without needing to pay for the damage your ogre friend will do with it). Essentially, in addition to just creating a big chunk of rock, you are also creating it with a massive amount of potential energy, calling for Create Energy. Whether this is Lesser or Greater is up to the GM - you might want to follow the damage guidelines from Destroy Matter.

At first, my idea was to add a surcharge to the ritual to account for such energy, but it occurred to me that the Create Advantage actually gives a more-or-less balanced matter-energy equivalence*, which works out to every 100 kJ of energy being worth 1 lb of matter. This should also be doable for explosives and batteries/power cells. Call it Potential Energy for objects created at a great height, Stored Energy for explosives and batteries/power cells.

After doing some unit conversions and simplifying, I've got the equations needed. For adding potential energy (such as creating a boulder 50 yards in the sky, as in the example ritual), use w*h*g/4000, where w is weight in lb, h is height in yards, and g is gravity in G's (1 on Earth), to determine how many lb-equivalents of energy you need to add. For explosives, this is instead 20*w*REF (every pound of TNT has around 2000 kJ in it). In either case, you must pay for both the weight of matter you create and the weight-equivalent of energy you add to it.

So, for that Bridge Breaker Charm, you're looking probably looking at Greater Create Energy (6) + Lesser Create Matter (6) + Lesser Control Magic (5) + Subject Weight, 50 tons (8) + Potential Energy, 1250 lb (5) + Range, 50 yards (8) + Duration, 10 minutes (1)**, for a total of 117 energy (39x3), which seems more appropriate.

For a Conjure C4 ritual that makes a 8 of the typical 1.25 lb blocks of C4 that lasts for up to a day, each 1.25 lb block of C4 contains energy equivalent to 35 lb of Subject Weight, so you're looking at Greater Create Energy (6) + Lesser Create Matter (6) + Subject Weight, 10 lb (0) + Stored Energy, 280 lb (3) + Duration, 1 day (7), for a total of 66 energy (22x3). You'd later detonate them either with normal blasting caps or a Lesser Create Energy ritual.

Optionally, a character can use Path of Matter in place of Path of Energy here (to make a character who can create boulders up in the sky but not fireballs). This is probably a trait worth [5] or so to do so for Potential Energy, but I generally wouldn't make it available for Stored Energy (if it is, maybe another [5] there). Optionally, only allow such characters to do this with rituals for which they either have a Grimoire or Ritual Mastery. Note the ritual that doesn't use Path of Energy is distinct from the one that does.

If you'd prefer, multiply the above equations by 100 (the first becomes 25* rather than /4) to get Potential/Stored Energy in kJ, and multiply the weight column of the Spell Modifiers Table by 100 to change from lb to kJ. Of course, that can result in some pretty big numbers to look at, so you might instead want to divide by 10 in each case to turn things into MJ. The table would change to this:
*E=mc^2 doesn't work here, because it means the 10 lb object you can make without adding any energy is worth something like 80 PJ, if I've done my math right. That's a bit over the output of the Tunguska event.

**Allowing "Momentary" to last long enough for the boulder to hit the ground doesn't seem like a stretch to me, but would only reduce the cost of the spell to 114 energy (38x3).

Re: [RPM] Potential Energy
 

The fundamental problem is that damage and weight are on completely different scales. Plausibly speaking, 'transform' and 'destroy' should be the same magnitude of effect, which means if we leave the weight table alone, we should actually have a damage table sort of like this
Separately from that, your 100 kJ figure is seriously high. My numbers mean that a 1 lb unliving object (4 hp) needs to be moving at 25 yards per second for the correct damage. That's an energy of 118J (not kJ).

Re: [RPM] Potential Energy
 

My system is scaling things based simply on energy, with no direct consideration for damage. The "1 lb = 100 kJ" figure is based off of Create, where matter is created as 10*(level squared) lb, while energy is 1000*(level squared) kJ.

I'm curious, however, as to how you came to your table. At first it looks like you're assuming an REF .0028 or so explosive for the 10 lb -> 2d figure (consistent with 0 energy for a 2d external explosive), but in that case 1,000 lb of it would do 20d damage, not 10d.
(EDIT: Upon a second review, it looks like you're more-or-less scaling things as +1 SSR to damage per +3 SSR to weight, which would be consistent with scaling explosive damage to the cube root of weight instead of the square root. Is that the case here?)

Crap, for some reason I was thinking kg*m/s2 was kJ, not J. I'll update my post shortly.
(EDIT: Updated)

Re: [RPM] Potential Energy
 

I started at a base of 'a damage effect should break an unliving object of equal mass' and then rebased it slightly to 1 lb = 1d. Thus, a 1,000 lb unliving object (40 hp) corresponds to a 10d attack (35 damage).

Re: [RPM] Potential Energy
 

Ah, so I was correct about you scaling by cube root, but was wrong for why you did so.

I'll give it some thought. It would certainly make things more concise than the current methodology I've suggested.

Re: [RPM] Potential Energy
 

I am very pleased to have spurred such an interesting debate.

I would imagine that making Gunpowder is not possible with lesser as it is an EXACT chemical mixture. Even if we give it that.
Using Greater Create Matter, three times with a duration of 1 day(39 energy each). Will create the ingredients of Blackpowder.

Now to my original intent.

Based on your critique (MUCH appreciated). Here is the renewed effort. I even went so far as to compute the Size of the beast.

Roughly, (Fast math) 81 tonnes Ft^2. and it stayed on the Ground.
EDIT: The reason to not decrease the Duration is the "secondary effects" of the such a large and heavy object interacting with the world.

Even if I choose not to use such a High weight. Controlling the shape to have a narrow base and wide top would still have the desired effect. Moreover making a multi-ton cylinder would be able to accomplish the goal also.

All this is fun. But the real truth is that a Transform matter at the base of a bridge would be all it would take to wipe it out. Which leads me to believe a clever Ritualist would never have to use more than 50 EP to cause utter destruction.

Ok. So update: The 3 rules Ask if it possible for A person could do it by themselves if it takes a long time and equipment, possible. If any are yes then it might be Greater, if 2 are yes. it is Most Likely Greater.
3 is clearly a greater or more.

Re: [RPM] Potential Energy
 

I would generally avoid any calculations of energy or energy-pressure generated by the falling object, just as I would avoid checking the shear modulus for the wooden beams or measuring the length of time of contact between the boulder.

The number one rule of RPM is that it is a Rule Zero system. If the intent of casting the spell is to inflict a bunch of damage, the vehicle by which said damage is delivered is irrelevant, especially if that vehicle will blink out of existence right after dealing that damage. As such, this is simply adding the Damage modifier to the spell with sufficient quantity and modifiers to represent a massive falling boulder. That it creates a massive falling boulder simply gives the option to cast this as a Create Matter effect instead of a Destroy Matter effect. I'd allow either for this sort of thing, personally, but others might be more stringent.

If the boulder is to stick around afterward for a while, I'd treat this as two effects: Create Matter (to make a semi-permanent boulder) and Destroy Matter (to trash the bridge/squash the troll/etc.).

Personally, I'd be curious to hear how PK would adjudicate this situation.

Re: [RPM] Potential Energy
 

Black powder isn't as complex as a crossbow, which I think is the simplest thing that calls for Greater. It would require a skill roll, just as making it from raw materials does - Explosives (Fireworks) or Chemistry at +4, or Explosives (Demolition) at full skill. Failing that, saltpeter, charcoal, and sulfur are certainly simple enough for Lesser Create, so three uses of that could make the raw materials, then you use normal skill and tools to make the explosive.

I don't think you need Area of Effect here - that's to affect multiple targets, not something you need to throw on to account for the large surface area of the thing you created. And, yeah, with that much weight you probably break through the bridge without needing to account for anything else. With Lesser Create Matter, you boulder is probably more an amorphous lump that approximates a cube (or cylinder, or whatever shape you're shooting for), but that should be fine, as you're going for sheer weight rather than aesthetics.

As a GM, I'm alright with not using the normal Damage modifier for something like this. I wouldn't have an issue with a ritualist at the top of a cliff using a non-damaging ritual spell to make a boulder and then shove it off to smash something below. With that in mind, there doesn't feel like there should be anything stopping a flying mage from using the same ritual to make an adjacent boulder, which would similarly fall and crush whatever was beneath. The issue is that doing either of those requires a difficult setup and precise aiming, while Ankhlord's ritual is basically just "Cast, Run, Smash" (or worse, "throw at the target, enjoy the view" if it's an ammunition Charm), because it adds in a vertical component to the ritual. As I'm alright with making an object at the same elevation as the character with a normal Create Matter effect, but higher elevation means higher potential energy, I opted for a surcharge for said potential energy.

Of course, if someone wants to use the normal Damage modifier for a similar ritual, like a rain of stones, I'm fine with that.

Re: [RPM] Potential Energy
 

If you made the rock at your altitude and just dropped it, that would probably require a Dropping roll to hit.

Re: [RPM] Potential Energy
 

Yep. This is overthinking the subject. Just add the damage modifier.

Re: [RPM] Potential Energy
 

I think we are on the same page here. If a player creates a boulder and then has to invest effort to make it inflict damage, that's no different than conjuring a sword that he has to then swing. But if the net effect of the only use of a spell is to insta-gib someone/something, it's time to pony up some energy for the damage modifier.

My main point was that going down the rabbit hole of trying to calculate potential energy and kinetic energy eventually leads deeper to contact times, looking up Young's moduluses (modulae?) and shear moduluses for various materials, solving for internal bending moments, and all other sorts of mathy things that make RPGs generally less fun when they become necessary to resolve in-game actions. I just suggest it's easier to adjudicate the spell as either "Roll to gather energy!" or "Sure, but you'll need a Destroy Matter effect and add appropriate Damage modifier. But yeah, do that and lets start making energy gathering rolls!"

Re: [RPM] Potential Energy
 

Note in the case of a flying mage (or one standing at the edge of a cliff and creating a boulder where ground isn't), I would still allow the character to create the boulder without issue, even though there is no further effort (aside from aiming, which may not matter with a sufficiently-large boulder) required to allow it to fall and strike something. Similarly, I wouldn't force the character to pay a surcharge if he created a boulder on a hill and let it roll down to crush whatever as at the base.

Now, I'm not saying that using the Damage modifier and treating the spell effect as fluff is Hurting Wrong Fun - I just personally think it would be more internally consistent to allow a character to use rituals like this in nonstandard ways, and once you have someone who can make a boulder 50 yards away from himself horizontally (perfectly fine with just the Range modifier added in), it makes sense to be able to do so vertically.

Hey, now, leave the Armor and Wounding Overhaul I'm working on out of this. :P
More seriously, it seemed appropriate to me to account for the potential energy of such a thing (or stored energy, for explosives), and when I looked up the math I found it was actually something that should be doable at the table, at least assuming you've got a calculator handy (which I'd expect most groups using RPM would as a matter of course). "Multiply weight by height, divide by 4000, look up on the Spell Modifiers Table" isn't that complicated (the worst part is "divide by 4000," but even without a calculator that's just "divide by 4, move the decimal 3 places to the left;" heck, if it's too much of a problem, get rid of the 1000 part - doing so just makes the base cost 4 energy higher than it "should" be). Explosives are even easier - "Multiply the TNT weight-equivalence by 20." Things get even simpler if I opt to instead go with Anthony's table, since that uses values you have to calculate anyway (collision/explosive damage), and with a slight adjustment follows the easily-memorized (and thus commonly used) Size and Speed/Range table (it even defaults to 2d being +0).

The point here is that the basic damage modifier in RPM is good for simple "I kill you in the face" spells, while allowing for a variant (that will ultimately result in a discount) for doing things in a more roundabout fashion - reconfiguring a "summon boulder" ritual to make a boulder up in the air, using personal skills to create explosives rather than just causing an explosion of energy, and so forth.

Re: [RPM] Potential Energy
 

If you conjure the boulder above them, this is essentially indirect fire, and probably will miss, without doing some kind of fire direction, or having an aiming sensor collocated with the boulder.

Re: [RPM] Potential Energy
 

Pretty much this. Why aren't you just using damage? Calculate the amount of HP a bridge has and go from there. Alternatively, figure it's weight and use Transform to turn it into air. I'd also give the bridge a bonus to its resistance roll equal to its total SM.

Re: [RPM] Potential Energy
 

The character would need to use a rangefinder (possibly a range-finding ritual) or relevant skill to figure out the distance from himself to the target, then summon the boulder 50 yards above that point. I had originally* written a response to your note of needing Dropping skill with something along the lines of "I'd be fine with requiring a skill roll to actually hit the target," but looks like I failed to paste that into my response to Humabout.

*(I very nearly did it again with my response to Ghostdancer, hence this response being a late edit here)

I like the idea of rewarding the character for using a more roundabout method of creating the effect.

As it stands, there are at least three mechanically distinct ways to use Path of Matter to destroy a bridge (or castle wall, or whatever) - External Damage to fling a chunk of matter at it (might be able to get away with Lesser Create Matter - the chunk doesn't need to be complex - and Lesser Control Matter - to fling it forward or make it fall incredibly fast to cause damage without need for build up, although I could see cause to boost the latter to Greater Control Matter), Internal Damage to make it break apart (Greater Destroy Matter due to the damage needed), or simply bypassing the need to damage it by reshaping it into a form that cannot support its own weight (via Lesser Transform Matter; Greater Transform Matter to transmute it to air might not be an ideal option here, as presumably the bridge would return to normal once Duration expired). Is it really so bad to add another option, conjuring a stone above a bridge and letting gravity do the work?

Re: [RPM] Potential Energy
 

It seems like too much work to add to the GM for the same exact effect - in three ways as you noted. Though you could also do something like Affliction + "Instant Death" (Powers, p. 118) too. This is the sort of stuff that can force a game featuring RPM to a screeching halt. I'm all fine with adding to a system if a) adds flavor b) doesn't inconvenience the player/GM or c) doesn't reduce fun. Adding complexity for the sake of complexity is not a good path to travel down with a free-form system in many cases. I think this is one of them. Just my two cents of course. But I think you'd find it difficult to GM. If you find this sort of thing fun...well, I'm not going to come to your house and stop you.

Re: [RPM] Potential Energy
 

You would also need TL5 math and the ability to specify the angle and distance of the boulder relative to yourself. Arguably this is Forward Observer* and Artillery (Bombs).

* Due to the lack of a fire direction skill.

It certainly is lot easier to just do damage.

Re: [RPM] Potential Energy
 

Or "Summon Miley Cyrus" and her wreckingball.

Re: [RPM] Potential Energy
 

Depends on what magic requires of you. You're assuming it's "Create the boulder (2900)^(0.5) (call this hyp) yards away in that direction, in the air at an angle of (cos-1)(20/hyp) from my position." In that case, yeah, that's going to call for some math, although TL5 seems it might be a bit much - Pythagoras gives us the first equation, and I suspect Archimedes would have been able to get close enough to the second to work here, and they were both around at TL 2 (although they may have been using TL 3 mathematics).
(faux EDIT: With tools, this should certainly be doable at TL 2 using a cross-staff or similar)

However, I'd expect it to be more "Create the boulder 50 yards above a point 20 yards in that direction," in which case all you need to work out is the distance to the target (and maybe the distance to the ceiling if you're indoors, but this isn't as useful of a spell indoors, as you can't use gravity to as much of an advantage). The question is the skill you'd use for determining distance - any ranged combat skill might be sufficient, albeit likely at a penalty (this calls for more accuracy than typically needed to hit with an arrow) and a Per-based use, as would Forward Observer or whatever a commander uses to designate the target of a volley of arrows (either FO or Per-based Tactics). Observation could also work. The larger the boulder, of course, the less accurate you need to be. A better bet, of course, is to just designate the bridge (or an article of clothing, for a human target) and use Greater Sense Matter to get the exact range (Greater Sense Matter + Range + Subject Weight would be sufficient here).

"Summon Ron Jeremy" could be comparable, and gets you a free use of Terror on those nearby, which might be more useful.

Re: [RPM] Potential Energy
 

Observed indirect fire is TL6 (it wasn't invented until the 1930s).

I wouldn't be inclined to allow targeting of arbitrary points in space in general, YMMV.

Re: [RPM] Potential Energy
 

Artillery fire is markedly more complex than how I see this working. As I understand it, the former involves adjusting propellant and angle of fire in order to get an arcing ballistic trajectory to land where you want it to. Worst case scenario, Summon Boulder requires setting the angle and distance where you want the boulder to appear, and as I noted I'm much more inclined to just require the character to set the distance and height.

Fair enough. I'm fine with it for Area Effect rituals/powers, so I'm fine with it here as well.

Re: [RPM] Potential Energy
 

Targeting the ground is a whole of a lot easier, especially if laws of sympathy and the like allow a symbolic link to a landmark or reference point. Points in free space aren't so easily defined.

Re: [RPM] Potential Energy
 

Oh, I see what you mean then. Yeah, depending on the metaphysics of the setting, I could easily see summoning something in the air rather than on the ground being markedly more difficult. Of course, I'm going to mangle your interpretation (sorry) and instead use that in place of potential energy to justify the surcharge for making the boulder appear in the sky rather than on the ground. In fact, I won't even have the character's elevation matter - what matters is how far from a proper reference point the boulder is appearing, and the heavier the boulder the more difficult it is to manage. This frees me up to use Anthony's progression instead (avoiding a calculation in the process, as you need to work out falling damage anyway), and keeps the Ritual using a single Path, although making matter form in the air might be appropriate as a Greater Control Matter effect. Once I've done that, it makes sense to do the same for explosives, but those I probably would still require Greater Create Energy (Lesser Create Energy can make small batches, however). As batteries/power cells don't do damage*, just use the 100 kJ = 1 lb (or more appropriately for RPM, 1 MJ = 10 lb) equivalence. Obviously, that's also Create Energy (or Restore Energy to recharge an existing battery), although I'm a bit hazier on where the Greater/Lesser divide would be - as much energy as what UT calls a D cell might be a good breakpoint.

While I'm at it, might as well define how to handle a Rain of Stones ritual, or a Summon C4 ritual that makes multiple smaller blocks instead of one giant one. For the former, that's arguably just Rapid Fire added on. Making multiple blocks of C4 instead of one big one is comparable, but you have the option of combining the blocks back together for a larger explosion. That said, considering the way RPM handles Explosive, it might not be a terrible idea to revamp RoF in a similar manner. Currently, +1 energy is +3 SSR to mass (which is +1 SSR to damage if using Cube Root of Destruction, which from what I understand scales explosives more properly than the normal square-root rule). If we instead have +1 energy be +2 SSR to the number of explosives made, you're giving up raw damage (~+2/3 SSR if you combine the two blocks of C4 together) for the convenience of having multiple explosives. Applying this back to making multiple smaller boulders might be similarly appropriate - instead of +4 energy turning your ~7,000 lb boulder (50d) into a ~70,0000 ton one (200d), it gives you 20 of the smaller ones, which either attempt to attack the same target or get spread out thanks to adding Area of Effect (as an Advantage, a 50d, RoF 20 attack costs as much as a 125d attack, so it's still arguably a net loss).

*Unless they're the type that explode, in which case use the greater energy cost between nominal energy content and explosive damage caused.