D&D 5E Giant Boulder vs. Immovable Rod

[Cap'n Kobold]

You mentioned that there is a Strength check to stop the boulder. How does the DC compare to that of the check required to move an immovable rod?

 

[jgsugden]

When in doubt, turn to the dice. Rather than do the math yourself, set a DC for an intelligence/investigation check and let the PCs try to figure out how to do it. If they fail, the bolder crushes the rod. If they make it, the rod holds the boulder (and is sacrificed?)

 

[Baron Opal II]

In the gaming I'm DMing, the heroes are exploring a trap filled dungeon, and have correctly guessed that there is a giant boulder trap at the end of a certain hallway (based on Rolling Sphere from the DMG, but upgrade because it's a 15-foot sphere instead of a 10-foot). They have an immovable rod, and plan to use it to block the giant boulder.

I don't just want to do a gotcha on my players. My goals are to have fun encounters and encourage clever gaming. With that in mind what would you do?

I think it's a great idea and you should just roll with it.

Err... I mean not roll with it. Or, roll with the idea of the boulder not rolling. Or dice. Because rolling with this means no rolling dice with the boulder which isn't rolling either.

Umm.. It's a fine idea!

 

[MatthewJHanson]

You mentioned that there is a Strength check to stop the boulder. How does the DC compare to that of the check required to move an immovable rod?

The DMG version asks for a DC20 Strength check to slow it by 15 feet (starts with a speed of 60). Since my bolder is bigger I'm probably going to increase the DC, but not sure by how much yet. That's vs a DC 30 Strength check to move an immovable rod.

When in doubt, turn to the dice. Rather than do the math yourself, set a DC for an intelligence/investigation check and let the PCs try to figure out how to do it. If they fail, the bolder crushes the rod. If they make it, the rod holds the boulder (and is sacrificed?)

I like the idea of making them roll. They all dump stated Intelligence though.
My current idea is to have the rod roll a Strength check vs the Boulder's DC to slow it down, and continue to do that each round until the boulder final stops (or reaches the end of the hall).

 

[MatthewJHanson]

2. If it moves (slides down the ramp) while continuing to exert 8000lb force on the bolder, you can figure out how far the boulder rolls before stopping as follows: Multiply the weight of the boulder by the sine of the slope. --> W*sin(slope) = F1, F1 is the force needed to support the boulder. Subtract this force from 8000 --> 8000-F1=F2. F2 is the force that will slow down the boulder. If it is negative the bolder can not be stopped and will in fact accelerate. If it is positive the deccelration of the boulder can be calculated as this force times 32 divided by the weight of the boulder --> F2*32/W=d. With the deceleration figured out you can calulate how far the boulder rolls before coming to a stop by squaring the velocity and dividing by twice the deceleration --> S=V*V/(2*d). The answer is the number of feet the boulder rolls before stopping. Note I did this using scalers, not vectors. If you use vectors the signs can change a bit, but the answer should be the same.

Thanks for this. If I read this right. Is the velocity in feet per second? I initially did it as 60, because that's the boulder's speed, but then it occured to me that "feet per round" is not a very common unit.

 

[auburn2]

i think a round is 6 seconds so 60 ft per round is 10 ft per second.

 

[Eltab]

I like "The huge boulder rolls down the ramp, smashes into the Immovable Rod and sprays fragments everywhere." - Roll opposed checks for rock and rod -
If Rod wins "The rock is broken into big not-round chunks that grind to a halt at the bottom of the ramp. The Rod looks ruined."
If rock wins "The core of the rock bumps up over the Rod and keeps coming down the hall. Big not-round chunks of rock lie all over the bottom of the ramp. You cannot see the Rod in the rubble."

 

[Umbran]

If it's already gathered momentum by running down a steep ramp, the corridor could be perfectly flat and it'll still smash the rod with an order of magnitude more force than its maximum allowance.

Well, it won't "smash" as in "destroy". Per the item description, the rod simply shuts off if you put too much force on it.

Oh, oh, wait! Here's what you do...

Get yourself a big honkin' wedge of really stout oak, possibly bound in iron bands if you can manage it. Set it in the center of the corridor. Place the immovable rod against the downhill side of the wedge, and activate it. Yes, there will be more than enough force to deactivate the rod, but not before that rolling ball - that is traditionally only just big enough to fit in the corridor - will have tried to at least a bit roll up the incline, gotten several inches of wood under it, and wedged itself solidly in the corridor, unable to roll down.

Meat splatters. Stone and bone crack. But wood will first compress first, and there's no room to fit it and the ball in there.

 

[Paul Farquhar]

I calculate that a roughly spherical 300,000 lb rock would have a diameter of at least 14 feet. So the answer is simple - the rock gets stuck, it is too wide to roll down the passage - passages wider that 10 feet are against dungeon building regs.

Joking aside, the more you scale up the rock the more clearance there is to avoid it, unless you make the tunnel circular in cross section simply standing aside would avoid such a huge rock. The widest point would be at 7', which is over the heads of most PCs.

 

[Ruin Explorer]

Well, it won't "smash" as in "destroy". Per the item description, the rod simply shuts off if you put too much force on it.

Oh, oh, wait! Here's what you do...

Get yourself a big honkin' wedge of really stout oak, possibly bound in iron bands if you can manage it. Set it in the center of the corridor. Place the immovable rod against the downhill side of the wedge, and activate it. Yes, there will be more than enough force to deactivate the rod, but not before that rolling ball - that is traditionally only just big enough to fit in the corridor - will have tried to at least a bit roll up the incline, gotten several inches of wood under it, and wedged itself solidly in the corridor, unable to roll down.

Meat splatters. Stone and bone crack. But wood will first compress first, and there's no room to fit it and the ball in there.

This seems very like the sort of solution my players would tend to come up with. You're right re: these balls typically being barely able to fit in the corridor - and yeah that would mean they were easy to jam up, and easy to predictably dodge - i.e. by laying down (or in this case, ducking slightly if tall). If they were bouncing down the corridor unpredictably, that'd be a different matter.

This is actually interesting because it reveals why in MMORPGs and the like, when you have boulder traps or similar, it's always multiple smaller boulders. One large boulder, if it could be avoided at all, would be utterly predictable and trivial to avoid once you knew about it (or even knew about that type of trap on general).

Joking aside, the more you scale up the rock the more clearance there is to avoid it, unless you make the tunnel circular in cross section simply standing aside would avoid such a huge rock. The widest point would be at 7', which is over the heads of most PCs.

Yeah exactly. This rock is so huge it's actually a lot less threatening than a smaller one would be. You'd almost have to intentionally get hit by it - your natural instincts would get you out of the way if it's as slow as is being discussed.

There's a Dex save to avoid damage, which I figure it diving into the corners.

Unless it's going incredibly fast, like, way more than 10ft/second, then they shouldn't need to save if the players actually say "we dodge to the side". If a player ums and ers about it, sure, an easy-ish save might be warranted, but the thing is 15' tall, so you barely even need to duck to avoid it, given it's a sphere, in a roughly square corridor, if you just move to the side, which, assuming the corridor is barely 15' wide and you're in the middle, takes under a second (just try walking 7.5'). If it's a total surprise and coming at level 30+ mph or something then a save might make sense, but 10'/second is 6.8mph.

This is kind of less of a trap and more of a feature.