D&D General Is this a fair trap?

[Lanefan]

From the OP:

the rope is in fact supporting a large stone block that sits in the ceiling above the pit (by passing through a hole in the middle of the block, over a hook/pulley that hangs from the true ceiling above the block, and then splits or is knotted into four strands which run to each corner of the block, thereby suspending it).

That means the rope would have to pass through the yellow mold on top of the block, as well as the block itself.

Indeed, but if I'm the trap builder I'm only putting the flammable chemicals on the lower part of the rope - between the GC and the boulder.

 

[Maxperson]

All I know is that after 7 pages of this trap, I now want to use it in my game.

 

[Lakesidefantasy]

Could an African swallow carry a coconut?

Sorry, I'll leave now.

 

[Lakesidefantasy]

How would you get a gelatinous cube into a 10 ft. x 10 ft. x 10 ft. pit anyway? Wouldn't it topple over and get stuck? Why is a gelatinous cube cubic? Could you lift one up? Would it hold it's shape? Is a gelatinous cube a liquid, a solid, or something non-newtonian? Is it a fluid, can it flow?

 

[pemerton]

So .. at 170 lb / cu ft, a 10 ft column of granite has about 12 psi (pounds per square inch) of pressure. A typical home water pressure is 50 psi, four times that. This pressure forcing GC material up seems to be quite low. Most of the effect would likely be from the initial impact.

You've calculated the static pressure: weight = 170 lb X 1,000 cu feet = 170,000 lb; pressure in psi = 170,000/ 14,400 [the surface area, in sq inches, of the block] = approx 12 psi as you say.

In this trap, though, it is the impulse pressure that seems relevant (qv: I might be able to balance a rock or brick on my head without being squashed; but drop that rock or brick on my head from a balcony and it might kill me!).

Impulse = change in velocity * mass / time of deceleration. Upthread we've established that the velocity of the block at impact is around 8 metres per second. The impact brings it to a stop. The mass in kg is around 77,000 kg. Let's say the deceleration happens over one tenth of a second (in "reality" it probably wouldn't be uniform; I've chosen this figure by Googling some collision web pages and using a generously long figure compared to actual collisions). Then we get impulse = 8 * 77,000 / 0.1 = 6,160,000 N. With a surface area of around 9 sq metres, we get a pressure of around 684,000 Pa. That's about 100 psi.

The more rapid the deceleration the greater the pressure. I don't know what sort of pressure is going to be necessary to splat a Cube. The spongier the Cube and the more deformation it can handle, I guess the greater the deceleration time and thus the less the pressure the Cube suffers. My attempt to Google up the physical properties of jellies wasn't very successful (the webpages and papers that came up weren't ones I could interpret for the purposes of this discussion); so I don't know what a "realistic" set of figures is for a Gelatinous Cube.

 

[Khelon Testudo]

No, it is a stupidly implausible and convoluted nonsense trap.

Welcome to D&D!

 

[Charlaquin]

Why is a gelatinous cube cubic? Could you lift one up? Would it hold it's shape? Is a gelatinous cube a liquid, a solid, or something non-newtonian? Is it a fluid, can it flow?

It’s in the name - it’s a gelatin. Or at least, a material with comparable qualities.

 

[Manbearcat]

I've sblocked the trap description:

Spoiler

A room contains a 10' cube pit in its centre, with a rope stretched taut between a ring in the bottom of the pit, and a narrow hole in the ceiling through which the rope passes.

There is treasure scattered on the bottom of the pit.

The pit is in fact filled with a Gelatinous Cube (which the PCs can't see either because the GM rules it is naturally invisible, and/or because it has an Invisibility spell cast on it).

The rope is covered in chemicals that (i) protect it from being dissolved by the Cube, and (ii) render it highly flammable. This (ii) becomes relevant should the PCs try to burn the Cube once they discover it (which may be a particularly attractive tactic because it is hard to attack the cube when it's down in the pit), because the rope is in fact supporting a large stone block that sits in the ceiling above the pit (by passing through a hole in the middle of the block, over a hook/pulley that hangs from the true ceiling above the block, and then splits or is knotted into four strands which run to each corner of the block, thereby suspending it).

If the rope burns, the block falls and splats the Cube over everyone, paralysing them. And on top of the block is a Yellow Mould which releases its deadly spoors if subject to a violent fall.

And here's the source:

Spoiler

Roger Musson, in Best of White Dwarf Articles v 1, p 41 (I don't know which number of the magazine it was first published in; but it's late 70s or maybe 1980).

Its not clear if the Yellow-Mold laden stone can be investigated without disturbing/displacing it. If this is low level and the only way to discover the Yellow Mold is by displacing the ceiling stone, then that is a garbage trap. If not, then its completely legit. It sounds like the architecture of the ceiling is such that you should be able to investigate it without it being disturbed. Otherwise, the mold dispersal would have issues getting to the PCs due to obstruction (even measured in microns, a mold spore is large enough to get obstructed).

If the GM is doing a sufficient job depicting the room's initial conditions, answering the player's questions, honoring their Exploration Turn action resolution, AND the block can be inspected without being interacted with...then its legit. I mean, this room's general layout screams Gelatinous Cube and Yellow Mold is a common trap/foil in B/X. Spidey sense of B/X players of any kind of experience will be tingling big time.

I'm assuming this is low level, but if its 7+ Expert, then you have the Thief climb the wall/ceiling > investigate > discover the mechanism and Yellow Mold. Levitate the Dwarf (that has the Ring of Protection!) > everyone out of the room > Dwarf kills the Yellow Mold with fire > passes the ST with its beefy Death Save! Dwarf somehow fails Death Save you Neutralize Poison (I treated YM as Poison).

Or, you have the Wizard summon a Fire Elemental after its discovered and the FE kills it while everyone leaves the room!

If its low level, the Thief climbs and discovers it while everyone gets the hell out of the room (the lot in life of a Thief). If this is the only way forward, you Levitate the Dwarf and have them kill the Yellow Mold with Fire while everyone leaves the room! After you deal with the mold, throw 5 * Flaming Oil in the pit > sac a length of cloth/rope from the door to the pit > light it > GTFO and let it do its work!

No, it is a stupidly implausible and convoluted nonsense trap.

I feel like you could describe upwards of 50 % of D&D traps that way!

 

[pemerton]

It’s in the name - it’s a gelatin. Or at least, a material with comparable qualities.

Do you enough about the mechanical properties of gelatin to help with my post 125 upthread?

 

[Lakesidefantasy]

It’s in the name - it’s a gelatin. Or at least, a material with comparable qualities.

Well, in that case, it would be impossible to get a mass of gelatine that size into the pit in one piece. It would tear itself to shreds.

But seriously, I think this research paper "On the Rheology of Cats" would help in our efforts to define the fluid properties of a gelatinous cube.

Oh dang! Now I'm doing it too!

(Sorry I don't know how to link things from my phone)

content/uploads/2017/09/rheology-of-cats.pdf