2022-03-21

Saved by Rolling Rock

Statue of Sisyphus rolling a rock

Our friend Baquies asked a question that I like very much: my OED Traps Digest has a 10' diameter rolling rock trap, which I basically lifted from Moldvay's Lost City adventure (module B4, area 39), and he probably swiped it from Raiders of the Lost Ark the prior year. 

Now, I stipulate that after the boulder comes to a rest, it can be pushed aside 1 in 6 by a normal man. Is that reasonable?

Looking at the Rockhound Resource website, one of the examples in a table there includes a rock that's "Car sized | ~10 ft" weighing in at at about 86,500 pounds (and I double-checked the calculation: assumes about granite density).

An answer to a Quora question (by Charles Collins -- no relation, I assume) says that the optimal coefficient of rolling resistance on level ground is 0.10 (for a car with inflated tires, or a plane bearing). For our rock this gives a critical force of 86,500 × 0.010 = 865 pounds.

Can a man push that much? That's something I suppose a gym could answer. I think it's pretty common to bench press 100 pounds or so. An answer on Reddit here says regarding a leg-press machine, "People can easily hit up to 400-600 pounds or more on these machines, I would design for a force of 1000 pounds to have some safety factor built in."

So maybe we should dial down our rock a little bit, say to about an 8' diameter -- thereby letting it roll a bit more freely down our standard corridor, and also reducing the weight by half, to about 44,300 pounds, with a critical rolling force of about 443 pounds. This seems to put it squarely in the range that a fit man could roll, if he had his back against one wall, and pushed with both legs.

(And our friend Seeker points out that the easiest rock to carve for this purpose would be limestone, so let's say that's the material, which increases the weight a small amount, i.e., by 2% or so.)

What are your thoughts on that? Will it make Sisyphus happy?

15 comments:

  1. 8' boulder is also nice, since it gives some extra measure of plausibility for folks who are able to avoid taking massive damage as it rolls down.

    Now lets figure out how long it would take for such a boulder to come to rest? :)

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    1. Even a 10' boulder should leave a space of

      5*(sqrt(2)-1) = 2

      feet at the bottom corners in a 10' corridor. Should be enough for characters to squeeze in if they lie flat.

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  2. Players ought to be expected to work together to some degree. I figure the boulder can be heavy enough to require 3 people pushing it before its moved.

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    1. This is a good point, too. Also: if the rock gets wedged up against a wall it may be unlikely someone can find space to leg-press it out. So in that case you'd need that teamwork happening (somehow).

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  3. Man, based on the title, I expected some talk about beers (I love me some Rolling Rock). Don't get me wrong, I love a nice big boulder trap as much as the next DM, but I'm still a smidge disappointed.

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  4. Is it perfectly spherical? Or is there a flat spot? If it came to rest there (seems likely) it would be much harder.
    Is the ground stone? Or dirt (displaceable dirt)? Again much harder. If it is on stone, what type of stone? How much traction can you get?
    Are bare feet better than boots for traction? Do halflings make good prybars?

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    1. I am assuming it's pretty much round, and on a level stone floor. For the rest, good questions!? :-D

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  5. NASA says a human male should be able to push around 800 Newtons.
    https://msis.jsc.nasa.gov/sections/section04.htm#_4.9_STRENGTH
    About 2/3rds of the way down the page.

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    1. Wow, what an awesome page of data, thanks for that! Looks like that's the case where they've got a leg pushing against opposing wall behind them, plus both arms pushing (result 175 pounds +/- 40 pounds or so).

      I love that they measure angle of elbow flex in unit of pi. :-)

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  6. Sorry for my question not being related to rocks (I'd probably judge on a case-by-case basis but this seems like a good baseline for sure), but have you ever examined the wilderness fortresses as seen in OD&D vol. 3 and DMG1e Appendix C? I've been trying to use Appendix C, but it seems somewhat incomplete or at least confusingly worded and I thought you might be the best person to answer what's going on there.

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    1. I agree there's something a bit odd with those rules and it's something I've grappled with and asked around a bit myself. The AD&D Appendix C seems even more odd to me (one of those awkward expansions from the original). In OD&D it's at least fairly simple: always a PC-type leader, body of human troops, one monster-y type guard cohort.

      Vis-a-vis our Wandering DMs show today, I was told by Mike Mornard (original player with Gygax) that the "sally forth" from the castle rule in OD&D came fairly directly from the event in Anderson's Three Hearts & Three Lions, plus Arthurian myth. In actual play it's a little hard to wrap my head around, as written.

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    2. Yeah, I ended up using the DMG for character-type and henchmen while ignoring the four troop types and numbers given, and using OD&D vol. 3 for garrison and monster-type, though that does mean a bit of fudging with some of the classes added later (assassins use the EHP monster-types and monks use Patriarch?). I'm definitely curious how the AD&D version developed, if we can ever find out.

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    3. There's a pair of old Judges Guild supplements that I use, pretty sure on DTRPG, "Castles Book" I and II, that I think are intended as big expansions of the OD&D rules. About 50 actual castle maps each to drop in, greatly expanded followers tables for each late-D&D class type (incl. clerics by the 3 alignments, thieves/assassins, monks have their own table). Pretty much in the pocket for that use-case.

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