Is Leomund's Tiny hut *truly* Immobile?

[Renduaz]

I've recently been having an interesting argument somewhere about the RAW of Leomund's Tiny Hut, specifically pertaining to the following stipulations in the description:

>" A 10-foot-radius immobile dome of force springs into existence around and above you and remains stationary for the duration."

The argument begun based on what would happen ( And some of you might know where this is coming from ) if someone were to cast the Tiny Hut on a moving ship. I said that based on the first, primary definition of immobile in particular ( incapable of moving or being moved. ), even if were to make leeway toward 'stationary', what would happen at best is that the caster and the moving ship would simply sail past the boundaries of the dome of force that sprang up around him in a few seconds, at which point the spell would dissipate, or at worst ( And this is even more accurate in my opinion ) - The hut would completely break the ship apart as it's rear section, which wasn't inside the 10 foot radius at the time of casting, would get completely raked against the hut's hemisphere in a tug-of-war game between the section of the ship that was inside the AOE that is still trying to sail forward and the area which wasn't, now being barred by the hut. Imagine a titan just driving down a wedge into the middle, basically.

The opposition retorted that I'm wrong and that it is 'implicitly understood' that the hut is "anchored" to some point of reference like a ground or surface. I of course told them that not only is it written nowhere in the spell itself, but there are spells which do refer to a ground or surface ( I.E Tenser's Disk ), but this one explicitly does not. You can cast it in the air or in space or anywhere you want to and it springs into existence around you. ( And yes, it does have a floor )

So then they asked if that means whenever anyone casts the hut on the ground in a planet which is rotating or orbiting a sun, whether the hut gets instantly jettisoned away from him as a completely immobile ( relative to the 3D space it occupies ) magical force dome that is not affected by gravity and is incapable of moving or being moved. I responded that if your fantasy D&D planet employs real earth physics ( which ****s up A LOT of spells, Teleportation for instance ), then yes, by RAW that is exactly what would happen, no matter how outlandish it sounds. The hut will only remain around you on a crystal sphere/planet that does not abide by those laws. ( I.E does not rotate, and either the sun orbits it or else some kind of magic/god is responsible for day night cycles )

So a major argument ensued, and I proceeded to ask - If we're going to rule that the hut can simply be carried by a moving ship, despite being immobile, which goes against everything that the spell is trying to prevent, namely that you won't be able to just bring it with you wherever you need it, then does that mean if I cast it while riding a horse, it's now moving with me on the horse? If I cast it while holding a stick, does the stick work as a frame reference? What happens to the hut on the moving ship if the ship gets destroyed? If a chunk of wood from the deck it was cast upon survives and floats on the water, does the hut keep moving along with it? If the hut now uses the sea as references, does it it move with the waves or does it, arbitrarily, not move at all? If a giant asteroid destroys the planet you're on, what happens with the hut and the caster inside it? Since it apparently responds to gravitational pull, does it begin to orbit the sun or spin around with the debris?

Basically, in order to justify ascribing "implicit understandings" to immobility and straying from the explicit RAW, you end up in a spiral of infinite, arbitrary, and probably contradictory adjudications in a bid for consistency, and none of it based on the actual spell. So as far as I'm concerned, the hut is exactly as I read it - an immobile dome of force with no anchors or surfaces or grounds whatsoever, except the space on which it was imposed. Even if that means that yes, if your D&D world is a perfect replica of earth, then you do get spirited away from it every single time you try to cast it, with the only fix being to make your physics fit RAW instead of making RAW fit our own universe's physics.

But maybe someone could offer a better insight.

[Unoriginal]

If you are standing still on a moving ship, without moving an inch yourself, are you immobile?

[MaxWilson]

So then they asked if that means whenever anyone casts the hut on the ground in a planet which is rotating or orbiting a sun, whether the hut gets instantly jettisoned away from him as a completely immobile ( relative to the 3D space it occupies ) magical force dome that is not affected by gravity and is incapable of moving or being moved. I responded that if your fantasy D&D planet employs real earth physics ( which ****s up A LOT of spells, Teleportation for instance ), then yes, by RAW that is exactly what would happen, no matter how outlandish it sounds. The hut will only remain around you on a crystal sphere/planet that does not abide by those laws. ( I.E does not rotate, and either the sun orbits it or else some kind of magic/god is responsible for day night cycles )

That doesn't follow. In real life, there is no preferred frame of reference[1], so you can define a reference frame in which objects resting on the Earth are immobile, or a reference frame in which objects resting on the Earth are moving thousands of miles per hour around the sun, and it doesn't matter which one you use because they both give the same predictions in every situation.

But if you're trying to make 5E physics work, you can just say the dome is immobile w/rt the center of mass of the local planetary body (and maybe it doesn't work at all outside the gravitational field of a planetary body). And that's a perfectly locally-valid reference frame.

Anyway, "real earth physics" has nothing to say about how Leomund's Tiny Hut should operate. Real earth physics has no problem if you just use a sensible definition of "immobile". Real earth physics is not opinionated about reference frames.

[1] That we know of. Obviously, if somebody discovered a phenomenon which ONLY works w/rt some specific reference frame, all of that would go out the window and we'd have to re-invent much of modern astrophysics to account for the new phenomenon.

[Renduaz]

If you are standing still on a moving ship, without moving an inch yourself, are you immobile?

Nope. You are in fact both moving and being moved, or in other words, the direct opposite of the definition. Do people use the word 'immobile' less rigorously in daily life? Of course, you're moving every time you breath too, right? But since it's a dome of magical force that we're speaking of here which can stop anything in it's path, then we need a *literal* definition. And this is the only one which the spell gives us.

That doesn't follow. In real life, there is no preferred frame of reference[1], so you can define a reference frame in which objects resting on the Earth are immobile, or a reference frame in which objects resting on the Earth are moving thousands of miles per hour around the sun, and it doesn't matter which one you use because they both give the same predictions in every situation.

But if you're trying to make 5E physics work, you can just say the dome is immobile w/rt the center of mass of the local planetary body (and maybe it doesn't work at all outside the gravitational field of a planetary body). And that's a perfectly locally-valid reference frame.

So would it or wouldn't it enable the casting on a moving ship, or moving horse, or the back of a Goliath that you're being carried around by while casting it? Also, while it might be a perfectly valid reference frame, it won't be perfectly valid RAW - There are no restrictions on casting the hut in outer space, in a plane, in the Astral Sea, or anywhere else.

Anyway, "real earth physics" has nothing to say about how Leomund's Tiny Hut should operate. Real earth physics has no problem if you just use a sensible definition of "immobile". Real earth physics is not opinionated about reference frames.

[1] That we know of. Obviously, if somebody discovered a phenomenon which ONLY works w/rt some specific reference frame, all of that would go out the window and we'd have to re-invent much of modern astrophysics to account for the new phenomenon.

And what's the sensible definition?

[DMThac0]

So would it or wouldn't it enable the casting on a moving ship, or moving horse, or the back of a Goliath that you're being carried around by while casting it? Also, while it might be a perfectly valid reference frame, it won't be perfectly valid RAW - There are no restrictions on casting the hut in outer space, in a plane, in the Astral Sea, or anywhere else.

is answered by this:

Nope. You are in fact both moving and being moved, or in other words, the direct opposite of the definition. Do people use the word 'immobile' less rigorously in daily life? Of course, you're moving every time you breath too, right? But since it's a dome of magical force that we're speaking of here which can stop anything in it's path, then we need a *literal* definition. And this is the only one which the spell gives us.

If we follow this logic then you would never be able to cast Leomond's Tiny Hut ever. The planets are moving in an orbital and rotational direction, you are standing on the planet, thus you are both moving and being moved. The only way that wouldn't be true is if your planet is the center of the universe, with no orbit and no rotation, and such a thing would "realistically" cause issues with gravity and other physics.

If, however, you pause for a moment and consider that larger bodies may be able to carry an object and that object is considered immobile then we can move forward with this discussion. An object reaching a certain size, mainly something able to carry Leomond's Tiny Hut, without causing impediment could then be assumed to work.

-Cast it on a wagon that is slightly bigger than the hut, 12' by 14' to accommodate entering and exiting the hut; No, as the object is barely considered "Large" by the game's standards.
-Cast it on a ship which has a deck that is 18' by 60'; yes as the object is multiple size categories larger and considered able to carry/haul/house cargo and people.

There is something to be said about using common sense when asking these questions rather than trying to argue something which, if you were to search for it, has been run into the ground multiple times already.

[Renduaz]

is answered by this:



If we follow this logic then you would never be able to cast Leomond's Tiny Hut ever. The planets are pulled by a gravitational force, you are standing on the planet, thus you are both moving and being moved.

You're talking about our own universe's physics. D&D Physics do not necessarily align, and gravitational exertion alone will not suffice to make the hut un-usable anyway. The hut is evidently not affected by gravity, hence why it doesn't fall down when you cast it in the air, much like Wall of Force ( which can be explicitly free-floating ). You can still use the hut or the wall though. All that matters is whether the "planet" ( prime material ) is moving or not.

If, however, you pause for a moment and consider that larger bodies may be able to carry an object and that object is considered immobile then we can move forward with this discussion. An object reaching a certain size, mainly something able to carry Leomond's Tiny Hut, without causing impediment could then be assumed to work.

-Cast it on a wagon that is slightly bigger than the hut, 12' by 14' to accommodate entering and exiting the hut; No, as the object is barely considered "Large" by the game's standards.
-Cast it on a ship which has a deck that is 18' by 60'; yes as the object is multiple size categories larger and considered able to carry/haul/house cargo and people.

There is something to be said about using common sense when asking these questions rather than trying to argue something which, if you were to search for it, has been run into the ground multiple times already.

Anything at all can 'carry' the hut because it's self-suspended, has no weight and requires no support. You could """"carry"""" the hut on the tip of a spear that's been wedged into the earth if you wanted to. The only thing you're saying right now is basically that you'd only allow the casting of the hut as long as it is above some object which is bigger in volume or diameter than it, which is not only arbitrary but actually makes no common sense at all.

If I take an 18' by 60' blanket and cast the hut while standing on it, apparently the hut will now start getting dragged along with the blanket when I start pulling it. If there's a hole in the blanket, is it still the blanket? What happens if there's a hole in the deck? Does the hut fall? Does it finally stay behind? None of this is common sense as soon as you start really thinking about it. Why? because of trying to turn the hut into something that it's not. The hut is an immobile dome of force which has no weight and doesn't fall down when a surface is whisked away from it. It's not an actual straw hut.

If we want to violate the strict definition here, then you'd have to really own it. Just say "The hut can be cast wherever I think that it should be available to players, and move whenever I think it should move, and that's the only rule." - Then you'd actually be able to exercise common sense. No really, the spell can only make sense either by treating the word immobile like gospel, or else by making judgement calls about what a 'reasonable' anchor is based on case-specific personal taste and DM'ing style. ( Which goes into Ruling territory )

[DMThac0]

https://www.sageadvice.eu/2016/06/26...needs-a-floor/

https://www.sageadvice.eu/2017/05/20...ncient-dragon/

It seems that you are intent on finding a way to say that you cannot cast the spell. I am not a fan of quoting Sage Advice, but I figure that the only argument you'll take as valid will be from the guys at WotC...

[Nifft]

I'd probably rule that the Tiny Hut requires a solid, stationary surface in which to ground itself.

The ship and the ocean would not qualify unless the ocean was becalmed and the ship was (relatively) stationary.

[Renduaz]

https://www.sageadvice.eu/2016/06/26...needs-a-floor/

https://www.sageadvice.eu/2017/05/20...ncient-dragon/

It seems that you are intent on finding a way to say that you cannot cast the spell. I am not a fan of quoting Sage Advice, but I figure that the only argument you'll take as valid will be from the guys as WotC...

Mike Mearl's answers are not official rules for a reason. Only Crawford is ( And Crawford contradicted Mearl's answers on several occasions ). Now, the second link is an interesting dilemma - Did Crawford imply that the hut would slide off because the force dome has friction, weight, and would be anchored to the dragon's back, or that it would slide off because it would just stay behind as soon as the Dragon flew away from it?

I don't know, maybe it was the former, but the former does not match with the spell's description.

[MaxWilson]

So would it or wouldn't it enable the casting on a moving ship, or moving horse, or the back of a Goliath that you're being carried around by while casting it?

Are you asking me as DM? If so, my answer is, "It's immobile with respect to the local planetary body. If you cast it on a moving ship you're inviting the ship and the hut to smash themselves against each other, probably damaging the ship and destroying the hut."

Also, while it might be a perfectly valid reference frame, it won't be perfectly valid RAW - There are no restrictions on casting the hut in outer space, in a plane, in the Astral Sea, or anywhere else.

If you want to define a reference frame that works in wildspace/Astral Sea/etc. you are free to do that, as DM. One logical way to do it would be to say, "When cast outside of a planetary gravitational field, it is immobile with respect to the closest crystal sphere."

And what's the sensible definition?

I gave you one just now. You're free to use a different one. The point is that it's not "THE sensible definition", it's "a sensible definition." You can pick whatever sensible definition you want and real-life physics doesn't care.

[Renduaz]

Are you asking me as DM? If so, my answer is, "It's immobile with respect to the local planetary body. If you cast it on a moving ship you're inviting the ship and the hut to smash themselves against each other, probably damaging the ship and destroying the hut."



If you want to define a reference frame that works in wildspace/Astral Sea/etc. you are free to do that, as DM. One logical way to do it would be to say, "When cast outside of a planetary gravitational field, it is immobile with respect to the closest crystal sphere."

Well, that's practical as a ruling. But personally I would stick with "The Hut is immobile, period. It cannot move or be moved, by anything at all". Only because it is the unadulterated wording that requires no further arbitrary decisions on my part. Still though, using the local planetary body does make for much less potential random decisions as opposed to the mess which ensues by allowing the hut to travel on ships simply because they happened to spring into existence above a deck.

[Tanarii]

The answer to this question is for the DM to another question:
Is the player trying to be a rules-abusing muchkin or a savvy tactical innovater?

If the former, disallow the frame of reference you find abusive.

If the latter, congratulate the player on finding an innovative use of the frame of reference.

Personally I know which side of the line I put the argument that spell wouldn't work on a planet rotating through space.

A moving ship could fall either way for either frame of reference.

Again, IMO a moving wagon moving the hut with it is pretty obvious where it belongs, in my book.

Other DMs will have their own dividing lines. Always is when it comes to "physics interactions extensions of spells.

[tieren]

We use as a frame of reference the map. If it is big enough to need a map to describe you can cast Tiny Hut relative to it.

If its a small fishing boat probably not. If its 4 story galleon with dozens of rooms, we would allow it for a long rest in a cargo hold.

It would have to be a massive wagon for that to ever work (are the rolling Ziggurats still a thing?)

Common sense prevails.

[iTreeby]

Have boats be recognized as their own frame of reference because the ocean gods decided that the game is more fun when playable.

[DrowPiratRobrts]

1) I want to rule with the hut moving with the ship, but I won't because I like the idea of someone casting it and then the ship coming to a halt and remaining anchored for 8 hours.

2) Speed is relative. If you put a ball on the ground it's immobile, but only because it's moving with the same velocity and rotation as everything else on earth. It's relative speed/movement in the system is nonexistent, but it is actually hurtling through space. This is why if you're in a car holding a ball and you throw it in the air, it begins to decelerate. It's no longer moving with the system because no force is pushing it. If you had a tall enough car you could throw it straight up and watch it eventually hit the back of the car.

3) I want to rule with the hut making the ship stay anchored since the spell isn't centered on a point, but I actually like the idea better that the ship is able to sufficiently contain the hut as someone described above. This is obviously not the case for a horse and probably wouldn't be for a wagon either with the possible exception that it's completely enclosed like a tractor trailer.

4) Just to drop a bomb really quick...what about oxygen? It's matter. It's an object. You wouldn't have enough oxygen inside the hut for 8 hours with a full party, so can oxygen pass through the walls? Though this is slightly different than the ship, it provides a similar train of thought. The atmosphere is a much larger system that actually contains the hut, so it is able to pass through. Also so you don't have your players sleep in a death trap and never wake up. Sometimes we have to bend the RAW a little bit to arrive at a logical conclusion.

[Sigreid]

I'm not that concerned with RAW or SA. My ruling would be that it would move with the ship. The line would be that it moves with a reference large and solid enough to be the ground for all intents and purposes. So a large sailing ship would work, a cloak, wagon or rowboat would not. I would probably draw the line at somewhere around the size of an inn.

[Ganymede]

Leomund is locked in place in reference to whatever battle map you are using.

Problem solved.

[tieren]

2) Speed is relative. If you put a ball on the ground it's immobile, but only because it's moving with the same velocity and rotation as everything else on earth. It's relative speed/movement in the system is nonexistent, but it is actually hurtling through space. This is why if you're in a car holding a ball and you throw it in the air, it begins to rapidly decelerate. It's no longer moving with the system because no force is pushing it. If you had a tall enough car you could throw it straight up and watch it eventually hit the back of the car.


4) Just to drop a bomb really quick...what about oxygen? It's matter. It's an object. You wouldn't have enough oxygen inside the hut for 8 hours with a full party, so can oxygen pass through the walls? Though this is slightly different than the ship, it provides a similar train of thought. The atmosphere is a much larger system that actually contains the hut, so it is able to pass through. Also so you don't have your players sleep in a death trap and never wake up. Sometimes we have to bend the RAW a little bit to arrive at a logical conclusion.

Tangent, but:

In 2) that is not the way real world physics work. Newton's first law of motion, the ball moving forward with the car at 100 mph would tend to continue moving forward with the car unless acted upon by an outside force (air resistance, etc...) its why a gnat in the car can fly from the back seat to the front seat even though gnats can't fly 100+ mph.

In 4) the answer is magic, the spell text explicitly describes the condition of the atmosphere inside the hut.

[DrowPiratRobrts]

Tangent, but:

In 2) that is not the way real world physics work. Newton's first law of motion, the ball moving forward with the car at 100 mph would tend to continue moving forward with the car unless acted upon by an outside force (air resistance, etc...) its why a gnat in the car can fly from the back seat to the front seat even though gnats can't fly 100+ mph.

In 4) the answer is magic, the spell text explicitly describes the condition of the atmosphere inside the hut.

2) It is the way real world physics work. I never said the deceleration happens quickly, but it absolutely happens. Ask a physics professor, they can explain it to you better. My friend argued about this with me for a long time as a kid and then in physics our teacher explained it. In layman's terms, basically while the ball is suspended in midair the molecules in the air of the car cannot exert enough force, due to their gaseous form, in order to continue the ball's forward motion. So the ball decelerates it's horizontal motion since it's being pushed with less force and it accelerates toward the ground due to gravity.

4) Fair point, but I was simply using that as an example why you can't take the spell too literally. I don't think the PCs should run out of oxygen and I made that clear.

[Gryndle]

I've recently been having an interesting argument somewhere about the RAW of Leomund's Tiny Hut, specifically pertaining to the following stipulations in the description:

>" A 10-foot-radius immobile dome of force springs into existence around and above you and remains stationary for the duration."

The argument begun based on what would happen ( And some of you might know where this is coming from ) if someone were to cast the Tiny Hut on a moving ship. I said that based on the first, primary definition of immobile in particular ( incapable of moving or being moved. ), even if were to make leeway toward 'stationary', what would happen at best is that the caster and the moving ship would simply sail past the boundaries of the dome of force that sprang up around him in a few seconds, at which point the spell would dissipate, or at worst ( And this is even more accurate in my opinion ) - The hut would completely break the ship apart as it's rear section, which wasn't inside the 10 foot radius at the time of casting, would get completely raked against the hut's hemisphere in a tug-of-war game between the section of the ship that was inside the AOE that is still trying to sail forward and the area which wasn't, now being barred by the hut. Imagine a titan just driving down a wedge into the middle, basically.

The opposition retorted that I'm wrong and that it is 'implicitly understood' that the hut is "anchored" to some point of reference like a ground or surface. I of course told them that not only is it written nowhere in the spell itself, but there are spells which do refer to a ground or surface ( I.E Tenser's Disk ), but this one explicitly does not. You can cast it in the air or in space or anywhere you want to and it springs into existence around you. ( And yes, it does have a floor )

So then they asked if that means whenever anyone casts the hut on the ground in a planet which is rotating or orbiting a sun, whether the hut gets instantly jettisoned away from him as a completely immobile ( relative to the 3D space it occupies ) magical force dome that is not affected by gravity and is incapable of moving or being moved. I responded that if your fantasy D&D planet employs real earth physics ( which ****s up A LOT of spells, Teleportation for instance ), then yes, by RAW that is exactly what would happen, no matter how outlandish it sounds. The hut will only remain around you on a crystal sphere/planet that does not abide by those laws. ( I.E does not rotate, and either the sun orbits it or else some kind of magic/god is responsible for day night cycles )

So a major argument ensued, and I proceeded to ask - If we're going to rule that the hut can simply be carried by a moving ship, despite being immobile, which goes against everything that the spell is trying to prevent, namely that you won't be able to just bring it with you wherever you need it, then does that mean if I cast it while riding a horse, it's now moving with me on the horse? If I cast it while holding a stick, does the stick work as a frame reference? What happens to the hut on the moving ship if the ship gets destroyed? If a chunk of wood from the deck it was cast upon survives and floats on the water, does the hut keep moving along with it? If the hut now uses the sea as references, does it it move with the waves or does it, arbitrarily, not move at all? If a giant asteroid destroys the planet you're on, what happens with the hut and the caster inside it? Since it apparently responds to gravitational pull, does it begin to orbit the sun or spin around with the debris?

Basically, in order to justify ascribing "implicit understandings" to immobility and straying from the explicit RAW, you end up in a spiral of infinite, arbitrary, and probably contradictory adjudications in a bid for consistency, and none of it based on the actual spell. So as far as I'm concerned, the hut is exactly as I read it - an immobile dome of force with no anchors or surfaces or grounds whatsoever, except the space on which it was imposed. Even if that means that yes, if your D&D world is a perfect replica of earth, then you do get spirited away from it every single time you try to cast it, with the only fix being to make your physics fit RAW instead of making RAW fit our own universe's physics.

But maybe someone could offer a better insight.

by your same reasoning it becomes a WMD by the fact that it doesn't move with the planet. when in doubt don't ascribe physics-nullifying powers to a spell that would be above that spell level.

[lperkins2]

So, the short answer is it's up to the DM.

If the local cosmology uses the Ptolemaic system, then sure, you can say it must be perfectly stationary. You can also always define it as stationary to the largest nearby gravity source. This is a subset of the general question about spell reference frames, which always comes down to the local cosmology. For example, if in the local cosmology, the planet moves roughly similarly to how Earth moves, then teleport, if it does not sync your velocity to the destination reference frame, is basically an instant death sentence if you try to go more than a fairly short distance away. Other affected spells include Gate, Plane Shift, Glyph of Warding, and a large variety of others. There isn't a right answer to the question, but it is a good idea if the answer is consistent across related spells, and matches with the cosmology of the world. Mismatches can generally be weaponized by smart PCs (like casting L'sTH in front of a the mast of a ship cripple it).

[KorvinStarmast]

Have boats be recognized as their own frame of reference because the ocean gods decided that the game is more fun when playable.

If you have ever landed an aircraft on a ship at sea, you will likely agree with me that a ship at sea, while moving as compared to the surface of the planet, is certainly able to be a frame of reference such that I can (and did many times) make a successful landing on it.

To answer the OP's question.

While I appreciate your muse in exploring these kinds of questions, I also think that you are overthinking it.

For example, if the ship is large enough for a large or huge flying creature to land upon without upsetting the stability of the ship, you have a size range to work with in choosing "yes, big enough" for hut or "no, too small" for hut.

Large creature carries one medium creature easily for flying, in D&D 5e pseudophysics.
Manticore, Nightmare, Hippogriff, Pegasus, Giant Vulture, Giant Eagle, Giant Owl, WYvern, Gynosphinx / Androsphinx, Griffon, Efreeti, Djinni, Air Elemental. All Young Dragons. Pit Fiend, Horned Devil. Bone Devil. Vrock. Chimera. The Large Celestials.

A huge creature carries a lot more, up to as many as fit into the hut.
All Adult Dragons. Roc. Some Huge Demons like Balor.

A ship that can handle the huge one landing on the, no worries.
A ship that can just handle the large ones landing without upsetting stability, probably too small.

Somewhere between those two sizes of ship is a nice range of "yes, big enough and stable enough for LTH."

That's one way to look at it.

Alternatively:
LTH is about the same size as a small to medium sized helicopter.
If the ship is about the size of a yacht that can land those helicopters, then it's probably big enough for LTH.

[JackOfAllBuilds]

Sea sickness and “getting your land-legs” is a thing, dealing with motion and frame of reference. Google MdDS, the continued rocking after finishing a cruise.

On the backs of creatures obviously should not be allowed, but any inanimate vehicle large enough to be considered a building or structure with either/or both multiple floors or rooms. A siege tower is a tower on wheels. I can put a tiny hut on a tower.

(Simplest reasoning: “could I build a hut here?” Deck of a barge: yes. Dragons back: no.)

[stoutstien]

Other spells to consider with how you rule this
Glyph of ward-
Aoe concealment- fog cloud, silent image, darkness, and so on.
Wall spells-
Teleport circle-

[theMycon]

2) It is the way real world physics work. I never said the deceleration happens quickly, but it absolutely happens. Ask a physics professor, they can explain it to you better. My friend argued about this with me for a long time as a kid and then in physics our teacher explained it. In layman's terms, basically while the ball is suspended in midair the molecules in the air of the car cannot exert enough force, due to their gaseous form, in order to continue the ball's forward motion. So the ball decelerates it's horizontal motion since it's being pushed with less force and it accelerates toward the ground due to gravity.

Assuming the windows are closed & the AC is off, it is absolutely not. This is very easy to test. Tape a pendulum to the roof inside a car moving at a constant rate*. Or hold a piece of paper by the top. Or however you want to cancel out the force of gravity so you can continue the test indefinitely. You will see the object in question remain stationary relative to the car. If the car's a closed system, there aren't forces acting on its horizontal velocity, so there's no acceleration or deceleration. Like Tieren said, this is a straightforward example Newton's first law of motion.

If your physics teacher actually said something like that, (s)he was wrong in enough ways that they have no business teaching physics. A little bit of wrongness is expected of everyone- teachers are human too, that's why we have different teachers covering overlapping information in different courses- but "the air molecules cannot exert enough force to continue the ball's forward motion" is egregious.

Also, you did say the deceleration happens quickly when you said

it begins to rapidly decelerate.

*Don't drive while experimenting. Both activities deserve your full attention, and there's easier access to open space from the back seat anyway.

[Tanarii]

and then in physics our teacher explained it.

Either your teacher was wrong, or he was telling you something different from what you thought you were being told. That's not what happens and not how physics works.

If I had to guess, I'd say your teacher was talking about an accelerating vehicle.

[DrowPiratRobrts]

Assuming the windows are closed & the AC is off, it is absolutely not. This is very easy to test. Tape a pendulum to the roof inside a car moving at a constant rate*. Or hold a piece of paper by the top. Or however you want to cancel out the force of gravity so you can continue the test indefinitely. You will see the object in question remain stationary relative to the car. If the car's a closed system, there aren't forces acting on its horizontal velocity, so there's no acceleration or deceleration. Like Tieren said, this is a straightforward example Newton's first law of motion.

If your physics teacher actually said something like that, (s)he was wrong in enough ways that they have no business teaching physics. A little bit of wrongness is expected of everyone- teachers are human too, that's why we have different teachers covering overlapping information in different courses- but "the air molecules cannot exert enough force to continue the ball's forward motion" is egregious.

Also, you did say the deceleration happens quickly when you said

*Don't drive while experimenting. Both activities deserve your full attention, and there's easier access to open space from the back seat anyway.

Even with the AC off and the windows closed, you're not in a pressurized vacuum. When the AC is off in every car I've been in and you're going 70 mph you can still feel some air coming through the vents.

Not sure why I put the word rapidly there. That was a mistake. I'm not trying to argue that it's a noticeable change to the naked eye. And using a pendulum or holding a piece of paper is different because it's still tied to the system like parasailing or water skiing. My point is, there is actually air resistance even if it's minimal. I agree with what you're saying, but our assumptions about the car were not the same.

[Tanarii]

That's not how physics works.

[Joe the Rat]

If you really need to lock things into physics, mobility is always in reference to the local gravity well, which will be the planet. If you are spacefaring, consider researching a variant where you can explicitly fix your point of reference, because if you're in orbit and fussing about absolute motion, you might as well bring in relativity, which messily quashes absolute anything about spacetime.

If you're Spelljamming, you already know about the gravity planes on the vessels, which can serve as an anchor reference.

Me, I take a narrative approach. What is the ship? Yes, it's a vehicle, but as you travel, it is also your immediate (man-made) terrain. It's your point of reference, and random encounters are coming to you as much as you are coming to them. If your vessel is smaller than the hut, then the hut defines the terrain, and renders the vehicle immoble (and likely contained) for the duration. A good way to stretch your legs while piled in a life raft.

[DrowPiratRobrts]

Leomund is locked in place in reference to whatever battle map you are using.

Problem solved.

Many of us don't use grids/maps to play 5e though, so this is a good solution for some but not universal.