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5E Giant Boulder vs. Immovable Rod

MatthewJHanson

Registered Ninja
Publisher
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'm considering letting it just work because its called an immovable rod, and because I like to reward smart play.

The thing is though, that according to the book, the immovable rod actually has a weight limit of 8,000 pounds. I did some quick calculations, and a 15 foot diameter sphere of granite should weigh nearly 300,000 pounds.

That's way more than the rod can hold, but I also figure the stone wouldn't be putting its full weight limit on the rod. I'm pretty sure there's a way to calculate the force of the boulder on the rod, but my physics skills are rusty. (Also intuitively I feel like the current speed should factor into it, not just the acceleration).

Both the rod and the sphere can be thwarted by Strength checks, so I'm thinking about having them make some kind of opposed check against each other or something like that.

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?
 

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Oofta

Title? I don't need no stinkin' title.
Can they stop the boulder before it starts to gain momentum? Can they use the rod to deflect the path?

Because as you said, an immovable rod isn't really immovable, it's just difficult to move. At a certain point you'd need a wall of force or similar.
 

aco175

Hero
You could have the rod explode when it slows the rock down, granting them advantage to saves to get out of the way. If that is not enough you can have the rock do half damage and even reward another item at some point.
 

Umbran

Mod Squad
Staff member
So, you get into the question of how someone got a 300,000 pound rock anywhere. The largest stone at Stonehenge is about 30 tons, for comparison.

Presumably that rock has to roll down a slope. How steep is that slope?

Assuming the rock is not yet moving when they place the rod, the weight the rod has to support should be the total weight times the sine of the angle of the slope from horizontal.

so F = W * sin (angle)

For them to be able to stop it with the rod this way, the slope must be under 2 degrees. Of course, at this angle, you could mosey along in front of it, because it isn't accelerating at a high rate...

Alternatively, don't even try to use real weights of real rocks, because, honestly a 300,000 lb rock is kind of absurd.
 
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Umbran

Mod Squad
Staff member
Also... I guess they haven't thought of this, but they probably don't have to stop the boulder at all!

A spherical boulder in a square corridor leaves room in the corners! At that size, the PCs can lay down on the floor along the walls, and let the boulder just roll on by!
 

aco175

Hero
Alternatively, don't even try to use real weights of real rocks, because, honestly a 300,000 lb rock is kind of absurd.
You know that the evil mage with too much cash on hand and made the death trap dungeon just to test adventurers and his monster regeneration spell may have just used a wish to put the rock in place. ;)
 

Umbran

Mod Squad
Staff member
... may have just used a wish to put the rock in place. ;)
He needs a wish every time that trap gets set off to get it back up there. So, has that trap ever been tested?

Note this about wishes in 5e:

"The stress of casting this spell to produce any effect other than duplicating another spell weakens you. After enduring that stress, each time you Cast a Spell until you finish a Long Rest, you take 1d10 necrotic damage per level of that spell. This damage can't be reduced or prevented in any way. In addition, your Strength drops to 3, if it isn't 3 or lower already, for 2d4 days. For each of those days that you spend Resting and doing nothing more than light activity, your remaining recovery time decreases by 2 days. Finally, there is a 33 percent chance that you are unable to cast wish ever again if you suffer this stress. "

So, think about how many times he has to cast Wish to make his dungeon, and keep it plausible.
 

auburn2

Explorer
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'm considering letting it just work because its called an immovable rod, and because I like to reward smart play.

The thing is though, that according to the book, the immovable rod actually has a weight limit of 8,000 pounds. I did some quick calculations, and a 15 foot diameter sphere of granite should weigh nearly 300,000 pounds.

That's way more than the rod can hold, but I also figure the stone wouldn't be putting its full weight limit on the rod. I'm pretty sure there's a way to calculate the force of the boulder on the rod, but my physics skills are rusty. (Also intuitively I feel like the current speed should factor into it, not just the acceleration).

Both the rod and the sphere can be thwarted by Strength checks, so I'm thinking about having them make some kind of opposed check against each other or something like that.

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?
From a physics point of view you have two things at play here momentum and force.

First force: The 8000lbs is a force, so we will start with that - it is easy to figure out how much it is holding by multiplying 300,000 by the sin of the grade of the slope ... so some simple math - a 300,000lb bolder will exert 8000lbs of force at rest against a 1.6 degree slope. So if the ground is sloped more than 1.6 degrees the rod can't hold it at rest, if it is less than 1.6 degrees it can hold it.. That is the easy answer, but it does not consider if the boulder has a rolling start.

The force at rest is the only thing you can calculate with straight numbers and weight. If the bolder is rolling there is momentum, specifically it is equal to the speed of the bolder in feet per second times the weight of the bolder (300,000lbs) divided by 32. By one of Newton's laws (second law I think) the force exerted is equal will equal the time-rate change of angular momentum. You nhave to know how fast the boulder is rolling to continue here. You also have to decide, 1. do any peices come flying off the boulder in the collision (I could argue the boulder just smashes when it hits the rod, rewarding players for thinkning of it) 2. if the immovable rod snaps, moves or bends when 8000lbs is exceeded. If peices come flying off the boulder when it crashes into the rod but it keeps moving then the remaining weight is what we will deal with.

1. If it snaps it can't stop a rolling boulder.

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.

3. If it bends you need to figure out the strain performance of the moveable rod and use this to calaculate a moment applied to the boulder, translate this into a force depending on the alingment between the rod and the boulder, integrate this from 0 to 90 degrees of bend to determine if it stops the boulder or just bends down and the boulder rolls over it. There are a lot of numbers you are going to have to make up to do this.
 

MatthewJHanson

Registered Ninja
Publisher
So, you get into the question of how someone got a 300,000 pound rock anywhere. The largest stone at Stonehenge is about 30 tons, for comparison.
Since you asked, it's actually a regional effect of a variant black dragon. While most black dragons make water foul or vegitation hostile, this one makes traps spontaneously appear.
A spherical boulder in a square corridor leaves room in the corners! At that size, the PCs can lay down on the floor along the walls, and let the boulder just roll on by!
There's a Dex save to avoid damage, which I figure it diving into the corners.
So if the ground is sloped more than 1.6 degrees the rod can't hold it at rest, if it is less than 1.6 degrees it can hold it
The boulder comes down on a ramp that drops from the ceiling, which will deffinatly be more than that, closer to degrees. I haven't decided the exact slope of the actual hallway yet, but between one and two degrees sounds pretty good, so that's right on the line.
 

MarkB

Legend
The boulder comes down on a ramp that drops from the ceiling, which will deffinatly be more than that, closer to degrees. I haven't decided the exact slope of the actual hallway yet, but between one and two degrees sounds pretty good, so that's right on the line.
The 1.6 degrees maximum is to prevent the boulder from starting to move in the first place. 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.
 
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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

Legend
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?)
 

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

Registered Ninja
Publisher
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

Registered Ninja
Publisher
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.
 


Eltab

Hero
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

Mod Squad
Staff member
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.
 

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.
 
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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.
 

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