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The Red Bandits have boarded the train, hidden among the other passengers. They wait an hour or two - to be sure the train is far from any city - then, on the boss's signal, they all pull out their weapons. The boss shouts, "Everybody down! This is a robbery!"

As they thieves rush from car to car, robbing each passenger of jewelry and loose coin, our adventurers realize they need to do something or lose their recently-won loot. The rogue pulls out an Immovable Rod, places it against the car's door to bar it shut, and presses the button.

What happens?

Does the Rod use the ground (or the Prime Material Plane) as its reference frame for "unmoving"? Does the Rod stay still relative to the train car? Does it stay in motion in a straight line, which works fine (albeit with some shaking) until the train tracks turn?

Obviously this ultimately falls to DM discretion, but is there any rule or description that leans toward a particular answer? (I've tagged it for 5e, but I'll take an answer from any edition.)

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    \$\begingroup\$ There was an amusing thread on 4chan's /tg/ about the ramifications of being able to specify the reference frame of an Immovable Rod's immovability about seven years ago. suptg.thisisnotatrueending.com/archive/1709686 \$\endgroup\$ – JAB Oct 15 '15 at 16:15

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Per the rules, I think the Rod isn't moving and the train is. The rod flies "backward" through the train, punching holes in doors and walls and people. Bad news all round.

But that's a lame outcome. The party is trying to find creative solutions to problems, and as the DM, we want to reward them for doing that. Tell the rogue: "Ordinarily, the rod would align to the reference frame of the ground. It sounds like you're trying to align it to the reference frame of the train floor instead. What skill are you using to do that?"

You wrote that you'd take an answer from any edition, so I'll suggest that this is a great ad-hoc use of the Use Magic Device skill from 3.5e or Pathfinder. On a good roll, the rogue figures out how the rod is identifying its frame of reference, and manages to trick it into using the train floor as its frame of reference instead of the ground. On a bad roll, the Rod goes out of control as before, but the party at least understands why their plan didn't work.

If the group is playing 4e or 5e, there is no Use Magic Device skill, so you'd probably have to call for an Arcana check instead.

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    \$\begingroup\$ +1 for the idea that you might be able to figure out how the rod identifies it's frame of reference. I would be very careful with allowing that to work in general though since if you could manage that in general it would SOOOO break the game.:D \$\endgroup\$ – DRF Oct 15 '15 at 10:02
  • \$\begingroup\$ Chatty comments deleted. Reminder: comments aren't for chatting. \$\endgroup\$ – SevenSidedDie Oct 16 '15 at 15:19
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    \$\begingroup\$ 5e Thief (Rogue) does have Use Magic Device, PHB pg 97. \$\endgroup\$ – Nicholai Bush Aug 23 '17 at 13:27
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Nice question!

Magic and Physics

Here on earth we have this thing called physics that, among other things, explains why things fall down. There in our imagination we have magic that can do pretty much whatever we want; that's why its magic. However, in order to play D&D we start from the assumption that physics works over there in our imagination and that magic "breaks the rules" in some well defined way.

Let's just consider Newtonian mechanics (relativistic methods are not needed as we are not dealing with relativistic speeds).

The Rules

... the rod doesn't move, even if it is defying gravity. The rod can hold up to 8,000 pounds of weight. More weight causes the rod to deactivate and fall. A creature can use an action to make a DC 30 Strength check, moving the fixed rod up to 10 feet on a success.

"the rod doesn't move" doesn't mean anything unless you decide what it doesn't move relative to. Aside from the obligatory "That's up to the DM" lets try and think it through by considering our expectations in other situations.

I will assume that your campaign takes place on a rotating planet orbiting a sun similar to earth; it doesn't have to but that is the default expectation.

  1. You activate the rod while standing on the equator. Do you expect the rod to move west at a speed of 1,670 km/h? Probably not.
  2. Same situation but now you running north as fast as you can. Do you expect the rod to stop relative to you or the ground? Probably the ground.

So our expectation is the rod takes its idea of "doesn't move" relative to the planet (or plane) it is sitting on.

Applying that to the train, the rod stops and the train keeps moving. If the train weighs more than 8,000 lb then the rod immediately deactivates.

Perhaps a better idea is to get in front of a bandit and activate the rod at neck height?

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  • \$\begingroup\$ Would it compare its mass to the train, or only to the weight capacity of the surface it contacts? You could argue that the door it's barring weighs less than 8k pounds,and so it works fine \$\endgroup\$ – user47897 Mar 12 at 15:48
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To function as described, it must be understood to have a fixed reference frame. Otherwise it would be a Rod of Relative Motion: just set it in motion and activate it to keep it going as if on invisible rails. Clearly that can't be sensibly gained from the normative description of its function, so it must have a fixed reference frame.

To function as described, it must be the plane or planet, depending on cosmology. If the plane contains planets, it must not be the plane (or it would be an "Possible Instant Death by Rod Impaction Rod" as the Rod and planet suddenly diverge at extreme speed), so a planets-based cosmology must use the planet as its reference frame.

(Yes, a rotating reference frame causes some conceptual issues. Don't worry about it—it's magic. Just so long as we interpret it so that it normatively does what the book says it does, the exact process by which it pulls that off is immaterial.)

In a plane-based cosmology without orbiting planets, it's pretty simple to conclude that, to work as described, its reference frame must be the plane itself.

Whether planes or planets, the result in the same: the Rod's Immovability is effectively relative to the ground.

What this means for vehicles

Vehicles aren't special objects that create a special frame of reference more important than any other. We think of them as so, but only because that's conceptually easier than thinking about how fast we're going in delicate coordination with all this other stuff nearby that helped accelerate us so much.

In other words, the difference between a Rod on a train and a Rod in your hand as you're walking is nothing except degree of speed and enclosed-ness, and the Rod cares about neither of those things, if we take its description at face value.

So what happens when you activate it on a train is the same as what happens when you activate it while strolling along: it stops, and obstructs anything and everything that isn't also stopping with it, possibly causing damage.

How much damage? Well, that depends on the DM, so I won't speculate. But I will point out that the Rod is only mostly Immovable:

More [that 8,000 pounds of] weight causes the rod to deactivate and fall.

So if 8,000 pounds equivalent force press on the Rod, it concedes. I think a train, sturdily-built, would count as at least that much. A door would likely break first, but at some point the Rod will probably catch up on some sturdy part of the train that could transfer that much weight from the train's momentum before breaking, and deactivate the Rod. (Needless to say, the attempt to bar the door with the Rod would not succeed.)

The Rod can also be moved slowly on purpose, by a strong creature:

A creature can use an action to make a DC 30 Strength check, moving the fixed rod up to 10 feet on a success.

That sounds like less that 8,000 lbs of force? So a lot of weight just hanging off the rod won't make it move an inch, but less force with deliberate intent to move it will nudge it? I don't know, it's a weird item. But the point is! A DM could rule that a slow-moving (like, 10ft./6s) train could "make a Strength check" and if it succeeds, then the Rod stays on the doors without reducing them to splinters.

I don't know, it's a weird item, but a DM could extrapolate that ruling if they wanted to.

But most likely, an Immovable Rod on a train will just blow things apart and be generally chaos and bad.

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    \$\begingroup\$ +1 for vehicles not magically creating a new frame of reference. If they did, you could drive a cart under a mid-air rod in order to give it a new frame of reference, then drive the cart away with the rod hovering over it. \$\endgroup\$ – GMJoe Oct 14 '15 at 23:42
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    \$\begingroup\$ A creature with a Str of 30 (the minimum needed to make a DC 30 strength check) can drag along the ground around 8000lbs. So I guess such a creature putting all its strength against the rod might be able to push at it with close to 8000lbs, which would at least be a little consistent. \$\endgroup\$ – Erik Oct 15 '15 at 7:30
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    \$\begingroup\$ "Vehicles aren't special objects that create a special frame of reference more important than any other." I feel like there comes a point where a vehicle is big enough that this isn't true. Now, I'm not sure where the line is, but it's probably somewhere between Nimitz-class aircraft carrier and Puppeteer planet-ship. In any case, I think this is a situation where it's best for the DM to just eyeball what outcome is least stupid/most interesting. \$\endgroup\$ – Blue Footed Booby Oct 19 '15 at 18:26
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This answer explores implications of allowing a frame of reference smaller than "the world".

Here's some arguments for Immovable -- relative to the nearest frame of reference, which for my purposes will be defined as "fully enclosing frame of reference" -- a building, a closed vehicle, etc falling back to "the world" if you're outside.

One big advantage here is that you don't have to decide if "the world" is a flat plane or planets -- the rod just stays immobile relative to whichever one it's in -- nothing further is required.

That does have some implications that need exploring:

  • Q) What happens in the train mentioned in the OP?

    A) The device will remain immobile relative to the train. That means it's useful for things like blocking doors, but not for destroying the train

  • Q) What happens when the frame of reference changes?

    A) It doesn't change, the device remains immobile relative to the frame of reference it was activated in until it is deactivated or the frame of reference no longer exists (destroyed/other). In that case it deactivates

  • Q) What happens when I activate the device while it's in my zippered pocket?

    A) (Assuming that you don't have to physically touch magic items to activate them -- that's an open question in my book) Now you've got a rod in your pocket that won't come out until you deactivate it or your pocket is destroyed.

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  • \$\begingroup\$ +1 because the other answers were overthinking it and this one applies the KISS principle. \$\endgroup\$ – KorvinStarmast Oct 15 '15 at 12:38
  • \$\begingroup\$ Based on your first Q/A, if you run really fast and activate the rod, it will either keep moving all by itself and disappear into the sunset, or it will hover above your head, speeding up and slowing down relative to yourself (like an invincible weightless shield). \$\endgroup\$ – NibblyPig Jan 15 '16 at 11:49
  • \$\begingroup\$ @SLC But Gus was talking about a fully enclosing frame of reference. That would still be the Planet/Plane if you're running while holding the stick. So it would just stop - relative to the ground. \$\endgroup\$ – lucidbrot Jun 20 '17 at 6:39
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I find games like D&D work best if you throw modern physics away.

As an example:

  • Birds fly by being attached to the element of air. Feathers and the wings just apply that attachment.

  • When you throw something, it flies forward. When it runs out of forward, it falls.

  • Enclosed and mostly Encloses things, like Trains and Wagons and Ships, redefine stationary within themselves. Influence from outside (be it weather or bumps on the road or the like) can jar the stationary.

  • The "physics" of the pre-scientific method holds.

  • You fall while in the air because the Earth pulls the Earth in your body. There is no 9.8 m/s^2.

Following that internal logic, the people inside the car are clearly not moving. So the rod isn't moving either when it is held up against the door. So the rod holds the door shut.

Doing the same on a wagon, if it was "outside the wagon" enough, stationary would be the ground. If it was low enough, stationary is the wagon. If you tore the wagon apart, it would leave the wagon and stationary would be the ground.

A way to think about it should be "are you taking move actions to change position? If so, you are moving. If not you are stationary. If you are somewhat between the two, only then do things get fuzzy (and unpredictable)."

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The rules don't explicitly cover this situation. Furthermore, no earthly physical model predicts the existence of such an object or describes how it would behave, so we can't fully rely on science either. The DM has to make a ruling, but what should it be?

There are multiple valid answers

Like Euclid's parallel postulate, the "correct" ruling cannot be derived from prior knowledge. I propose that there are multiple reasonable rulings, and choosing one over another fundamentally changes the game being played - much like how the choice of parallel postulate fundamentally changes the geometry described.

Therefore, the correct ruling is the one that creates the game that the group wants to play.

I present three rulings, based on observations of groups I've played with:

Option 1: Magic obeys the intent of the user

Magic obeys the intent of its user, so long as that intent is within the scope of its function and consistent with its quiddity (its essence).

Applied to the train situation, it is clear that barring a door is within the purview of an Immovable Rod. It doesn't matter whether the door is on solid ground, on a moving train, on a ship's deck, or on a house on legs. The Rogue intends to bar the train door, so that's what the rod does. It is against the nature of an Immovable Rod to behave like a projectile, so it will never do that, even if activated on a moving train.

This ruling tends to create the most genre-appropriate game (for high fantasy), consistent with how magic "just works" in most fiction, and works well for groups that desire a narrative-focused and deeply immersive game. This doesn't require a medieval ruffian to puzzle out 20th-century physics (and all the underlying theory it's built upon).

Option 2: Magic obeys the laws of physics, as understood today

Magic follows all the rules of modern physics (as the players understand them), except in the specific ways it explicitly violates them.

Applied to the train situation, it is clear that we need to define which reference frame the rod is locked to. There is no inherently correct reference frame, so the DM would need to rule on that too, in whatever fashion feels most consistent with other use cases. The reference frame of the Earth/ground is the most obvious choice that satisfies expectations in a basic test case. Alternatively, the DM can reward the player who thought to ask the question by choosing the reference frame the players can best use to their advantage.

This ruling tends to create an environment where players are challenged to use their own scientific understanding to stretch the limits of the game world to devise clever solutions to problems. It's this sort of game where I've seen players discuss the third-order effects of detonating a Fireball, harvesting rust for creating thermite, and undermining the villain's fortress with an adamantine dagger.

[As the other answers attest, I suspect that most people who would think to ask this question are looking for this sort of game.]

Option 3: Magic is cool

Magic does whatever the DM thinks would be the most fun in that moment. Precedent and logic need not apply.

Applied to the train situation, the rod would surely subvert the character's expectation, likely causing substantial collateral damage along the way - while failing to protect the party from danger as intended.

This ruling is chaotic, but can be a lot of fun if the players don't take the game too seriously and aren't too attached to their characters.

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  • \$\begingroup\$ +1 for your different perspective: It's not about physics, per se, but about magic. It may not be gospel, but I really, really like your Option 1: as long as its consistent with the imovableness of the rod and doesn't violate an explicit rule, the caster's intent matters. \$\endgroup\$ – Wayne May 22 '18 at 19:02
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Physics, fundamentally, is a game we play where we figure out rules for how the world works. This whole 'quantum' thingy and 'relativity' and 'The Newtonian Schema' and all manner of other physical paradigms all come down to philosophical beliefs about the world. When I play D&D, I do not use modern physics, ever. I do not use modern biology. I do not use modern chemistry. I play my medieval fantasy with medieval science. People like to make fun of medieval science nowadays but, really, its a totally reasonable way of looking at the world that actually works much better in terms of predicting results in real life with the kinds of technology people in medieval Europe had access to than Newton's more theological approach. It's true that Aristotelian physics turned out to be wrong, but was still a physics, and the vast majority of the time using it will actually give you better results than applying the simplistic, physics 1A version of mechanics, because all of the factors the basic version of Newton's physics ignores move the actual results in real medieval life back towards Aristotle's explanation. In any case, this is the position from which the answer is given:

The Immovable Rod is immovable with respect to the unmoving foundations of the World (i.e. the Prime Material). It rips its way back through the train, cutting the robbers and anyone else in its path in two pieces. The train itself continues in its violent motion until the applied force (from the engine) is removed. Assuming the engine is in the front of the train, the Rod exits the rear from the perspective of the players and hovers above the track, while the train continues on and the surviving bandits give battle to the party (or surrender, depending).

I think there's a strength check or something to move the rod, so probably it should get moved during this, and the enemies should definitely get a save (probably Dex) to avoid damage/death. Assuming it's moved by the train, it will slowly float back to its natural position (i.e. where it was when the button was pressed) by natural motion.

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  • \$\begingroup\$ @GMJoe In Aristotelian Physics, there are two kinds of motion: Violent motion, when a force acts upon an object and forces it to move, and Natural motion, when an object moves of its own accord. Aristotle theorized that inanimate objects always moved by Natural Motion to the places they belonged, generally to the realm associated with their elemental being. The Immovable Rod moves by natural motion to the place it was activated because it thinks that is where it belongs (hence its strong resistance to being moved from there). So yes, it's a houserule, but only kinda. \$\endgroup\$ – the dark wanderer Oct 16 '15 at 3:35
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I like the idea of using a skill to operate the device in a new frame of reference, and I think a good solution to the frame of reference problem would be to say that the rod has its frame set upon creation as a part of its creation.

Since it wasn't created on the train, it won't have the train's frame of reference, and thus can't be used to bar the door — or requires some skill-use to use it the way they want. (In my opinion, this is the more fun idea because both outcomes are interesting, and they won't know which will occur until they try for it.)

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  • \$\begingroup\$ This appears to be a comment on an earlier answer, and would likely be better as such. \$\endgroup\$ – user2102 Oct 19 '15 at 13:38
  • \$\begingroup\$ Wouldn't setting the frame of reference at the time of creation make the rod start behaving strangely if, on a spherical planet, it moved more than a few miles from its creation site? What about when it's taken to another plane? \$\endgroup\$ – Martin Carney Oct 26 '15 at 22:44
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    \$\begingroup\$ Oh yeah... I guess this is why things are magic. How about this: you could have it set its frame of reference based on the gravitational forces its feeling. Since the planet or plane you're on dominates any other gravity on the rod, the planet or plane would then dominate the frame of reference of the rod at that time. (Also, @JoshuaDrake, I need 50 reputation to comment, so I was forced to make a new answer, sorry) \$\endgroup\$ – Shadow0144 Nov 2 '15 at 16:30
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The train would be destroyed, derail, and kill everyone on board most likely.

But if you think about it, there are lots of ways to destroy a train so it's not overpowered, it's just a very bad idea.

I would just ask the player are you sure you want to do this? You want to activate an immovable rod that inside a moving train?

My emphasis would be sufficient to let the player realise their mistake. Hopefully at least one party member has a drive for self-preservation and would tell them not to do it.

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  • \$\begingroup\$ I think that the OP was looking for some rules or at least flavor text to support any assertions in the answers. Can you add that to your answer? \$\endgroup\$ – user2102 Oct 19 '15 at 13:36
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    \$\begingroup\$ Afraid not, looking for rules in fringe situations is a task of common sense, otherwise we end up putting captured wizards in full plate armour again to stop them casting spells. \$\endgroup\$ – NibblyPig Oct 19 '15 at 15:40
  • \$\begingroup\$ Though "Don't do that!" can be an answer, I do not feel that it applies in this case. \$\endgroup\$ – user2102 Oct 19 '15 at 18:32
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The boring answer:

Weight threshold 8000 pounds ~ 4000kg ~ 40kN.
Rod cross-section sideways (let's assume a cylindrical rod, which deforms the pushed steel/iron/wood somewhat) 5cm diameter * 40cm length * 0.5 contact factor = 200 mm^2.
Contact stress ~ (40 * 10^3) / (200 * 10^-6) = 2 * 10^8 = 200MPa.
Steel ultimate contact strength: 400-1110MPa.

Conclusion: assuming the rod is stationary relative to ground and not the train, the rod will deactivate if it hits a solid steel wall.

Jammed against a door, it would probably rip the door and travel (possibly together with the door) through the wagons until it hits a sturdy steel wall.

Kinetic energy of rod relative to train: E = 0.5 m v^2 = 0.5 * 10^7kg * (60 km/h * 3.6)^2 ~ 233 GJ

Compare this to the 0.08GJ recoil energy of the Big Berta.

I am not sure if this means that things that can't stop the rod will simply be cleanly pierced/sliced or they will explode, killing everyone in the current train car. And any survivors will be deaf for life.

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  • \$\begingroup\$ What kind of door are you assuming? Also, for those of us who aren't engineers (either kind) and who, frankly and to our great shame, didn't get very far past algebra, can you put this in terms that explain the possibly explosive nature of the rod? I mean, that sounds awesome, but just math doesn't explain (to me, anyway) what's happening in the above equations. \$\endgroup\$ – Hey I Can Chan Oct 24 '15 at 21:32
  • \$\begingroup\$ You list 5cm diameter and 40cm length, both of which are reasonable, but your contact factor of 0.5 is not. That's 50% of the rod's surface in contact, if I understand correctly. Second, you changed units from cm * cm to mm^2, which is an order of magnitude difference. Third, you're still not calculating surface area right; surface area of a cylinder's sides (as opposed to its ends) is determined by its circumference, not its diameter. \$\endgroup\$ – Martin Carney Oct 26 '15 at 22:16
  • \$\begingroup\$ If I plug your values into this calculator, leaving Poisson's ratio and Elastic Modulus at the default values provided (since I'm not familiar enough with them to enter the "right" value), I get a max contact pressure of 212.3MPa. That's using the more precise conversion from lbs to N of 35.5kN. If I put in 40kN, I get 225.4MPa. \$\endgroup\$ – Martin Carney Oct 26 '15 at 22:32

protected by Oblivious Sage Oct 17 '15 at 18:47

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