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.