Any physicists/engineers able to provide input on this Reverse Gravity situation?

[gadren]

Hypothetically, say you could permanently apply the effects of Reverse Gravity centered on a single 1000 lb boulder (through research into the Permanency spell, a custom magic item, whatever). The boulder now "falls" upward. We have the boulder chained to a ship, and the ship, its contents, and the chains also weight exactly 1000 lbs. Will the ship achieve "neutral buoyancy" in the air? How hard would it be to move the craft forward, assuming we can adjust the weight of the ship with ballast?

[CGNefarious]

Assuming that the air resistance would apply equally both ways, which is reasonable, and the reverse gravity effect was only affecting the bolder, it should be relatively easy. Apply a horizontal force to the ship and (negating air resistance) it should continue moving until something else stops it. The way you're phrasing it is that the bolder is effectively negating the force of gravity on the ship. Then the only forces you need to look at are any horizontal forces applied, air resistance, and friction (which shouldn't be an issue). A simple wind based spell and a sail could be enough to propel your flying ship.

Of course you would need to be pretty precise in how much everything ways, or create a way to get some upward lift as well once you're moving.

[Alent]

Of course you would need to be pretty precise in how much everything ways, or create a way to get some upward lift as well once you're moving.

I would actually suggest using enveloping pit based ballast tanks for fine tuned control of displacement. It would look like a little like a trash compactor where the water is poured out of the ceiling mounted enveloping pit to descend and pushed into the enveloping pit to ascend.

This would more or less approximate a balloonet.

[nedz]

Well you couldn't do this with the Reverse Gravity spell since that effects an area not an object.

But, assuming you can get around this little technical problem, you would be better applying this to the top of the ship itself rather than a boulder, perhaps by using a plate of some kind instead, since that would make the vessel more aerodynamic and more stable.

Motion could be provided by a flight type enchantment or hitching up some flying draft animals — Air Elementals perhaps.

[Chronos]

Note that real-world airships do have wings, which can be adjusted to provide positive or negative lift. They're tiny in relation to the size of the ship as a whole, and couldn't possibly support all the weight on their own, but they let you account for slight imbalances in your buoyancy.

Extradimensional ballast, like Norren suggested, is a great idea, as it lets you re-use your ballast even after "dumping" it. You'll probably also want an Immovable Rod anchor. Immovable Rods would also allow you to "row", by fixing a rod at the front of the ship, and then hauling it to the back.

Actually, come to think of it, you don't even need extradimensional ballast (though everything's better with extradimensional spaces). Just move it between the positive and negative gravity zones.

Sails and wind spells might work, but natural winds would be problematic: With only one fluid to couple to, instead of two, you won't be able to tack against the wind.

Also, 1000 pounds is an extremely small ship. Even ten times that is pretty firmly in the "boat" category. You're going to want a bigger boulder. The downside, of course, is that both your boulder and your cargo will be contributing to your inertia, so your maneuverability will suffer.

[Alent]

Note that real-world airships do have wings, which can be adjusted to provide positive or negative lift. They're tiny in relation to the size of the ship as a whole, and couldn't possibly support all the weight on their own, but they let you account for slight imbalances in your buoyancy.

This depends on the airship. Most airships aren't really using wings as much as they're using vectored thrust. They have tailfin stabilizers, but they aren't control or lift surfaces. There are some hybrid lift almost-neutral negative buoyancy aircraft being experimented with, but generally most airships rely on positive buoyancy.


Actually, come to think of it, you don't even need extradimensional ballast (though everything's better with extradimensional spaces). Just move it between the positive and negative gravity zones.

I'm not sure this is a good idea. Functionally it works, and it provides more extreme adjustments, but it takes considerably greater energy to move. You're basically moving your ballast uphill both ways in the snow any time you need to change directions. Also, I would personally be using the entire reverse gravity effect radius for lift to maximize carrying capacity.

Although... since Nedz pointed out that you're enchanting a region of space to behave backwards, so you're going to need an envelope filled with stuff to anchor the spell to. You could use extradimensional balloonets in the envelope.


Also, 1000 pounds is an extremely small ship. Even ten times that is pretty firmly in the "boat" category. You're going to want a bigger boulder. The downside, of course, is that both your boulder and your cargo will be contributing to your inertia, so your maneuverability will suffer.

This a thousand times over. If you want this to be good for anything other than getting the main characters around from place to place, it's going to need a cargo capacity measured in tonnage.

[ExLibrisMortis]

I suppose it depends on what Reverse Gravity actually does.

It could turn the direction of the gravitational force by 180 degrees, within the spell's area, or
it could multiply the magnitude of the force by -1, within the spell's area, or
it could multiply the inertial mass of the things in the spell's area by -1, or
it could multiply the passive gravitational mass of the things in the spell's area by -1, or
it could multiply the active gravitational mass of the local planet by -1, and do any of the above everywhere besides the spell's area.

I'm not enough of a physicist to figure out which of these are totally superfluous doubles :smalltongue:. Magic is presumably able to affect any part of the equation, individually. Because magic is sensible like that (???).

Now, a negative mass (http://en.wikipedia.org/wiki/Negative_mass) boulder would be interesting. It would move in the opposite direction it was pushed in, for one. So you could have a troll with a big hammer on the bow of the ship, hammering the ship forward by smashing the front of your rock ship. Or, you could harness the power of runaway motion by mounting the negative mass rock on the back, and having the front of the ship attract it forwards (while the negative rock repels the front, also forwards).

[gadren]

Some good points.

I'd actually already considered the extradimensional ballast storage before this post, though my solution was the simpler one of lots of bags of holding and permanent unseen servants that move ballast from the bags to the ship and vice versa.

A friend pointed out that I could use a decanter of endless water and a bag of devouring.

What do you think, out of the current options in D&D, would be the most efficient ballast system?


Also, how bad is the inertia going to be on this puppy? Once it gets going, how hard will it be to stop?

[nedz]

Although... since Nedz pointed out that you're enchanting a region of space to behave backwards, so you're going to need an envelope filled with stuff to anchor the spell to.

Yes, the spell is one of the few parts of the rules which follow physics, at least in so much as Gravity is a function of space not matter.


Also, how bad is the inertia going to be on this puppy? Once it gets going, how hard will it be to stop?

With your design: Air resistance will slow you quite quickly. It will also limit your top speed.

[avr]

Also, how bad is the inertia going to be on this puppy? Once it gets going, how hard will it be to stop?
Imagine a car with no connection to the road and hence no brakes. At all. That hard.

If you have a flight enchantment that's fine, but it kind of negates the point of using reverse gravity. If you don't you're going to want parachutes and/or anchors or something. Also note that if your motive power is sails, tacking against the wind doesn't work without water under the hull.

[Jack_Simth]

A friend pointed out that I could use a decanter of endless water and a bag of devouring.

What do you think, out of the current options in D&D, would be the most efficient ballast system?


Also, how bad is the inertia going to be on this puppy? Once it gets going, how hard will it be to stop?
Well, a couple of things:

1) Unless you have some specific reason to hang the ship from a boulder, you probably don't want to do so. A Reverse Gravity spell directly beneath the ship does the job just fine, with the added bonus that you don't have to worry about being exact on your weight. For that matter, when you're lifting the ship itself in such a manner... as far as weight goes, you only have to worry about a few things: the structural integrity of the ship and thrust.

2) Ballast: If you're specifically needing ballast for some reason: Your basic lever & pulley system with weights in a Portable Hole or Enveloping Pit. Note, however, that your pulleys start pulling you down as soon as the attached weights are no longer supported by the bottom of the portable hole or enveloping pit - which means you need a lot of them, each of which moves just an inch or so, rather than a few of them that move a long distance (assuming rigid weights; if you have non-rigid weights, such as bags of sand, then things get more complex).

3) Inertia: You probably keep the exact same inertia, so you've got the mass of the boulder plus the mass of the ship. It'll be exactly as hard to stop as it was to start. You're going to need to be able to produce thrust with basically nothing to push against - so you're going to want some form of thrust generator - whether that's flying creatures with drag lines, Decanters of Endless Water, or Immovable Rods that you turn on and drag around. Without knowing actual thrust values from the various options, it'll be impossible to calculate.

[Elkad]

Assuming a thruster you can point in any direction, maneuverability wouldn't be terrible. At low speeds (where air resistance matters little) it's a hovercraft. Likes to coast, but not forever. Stopping takes almost as long as starting does. Tends to be "squirmy" when halted, you'll want to tie it to the dock.

At high speeds, you'll want a parachute for emergency stops. (probably two parachutes, you don't want all that extra stress on the boat<>boulder lines)

[Thealtruistorc]

It is generally NOT a good idea to ask physicists for D&D insight. Some seriously scary things can and will be conceived.

That aside, yes, it would work. However, something very interesting would occur if the tides ever dropped. Such an occurrence would (if my calculations are correct) leave your boat hovering in midair given the lack of acceleration upwards or downwards. You may want to check into that problem.

[CGNefarious]

Idea. Why not cast the reverse gravity spell on a small object (let's say a pebble) and keep it in a bag of holding. Make sure your ship has wings or something that allows it to glide a reasonable distance. When you want to increase your altitude you take your pebble out of the bag of holding. You ship will then be in the reverse gravity effect and rise. When you get to the desired height, put the pebble back into the bag of holding and glide for a while. Repeat to maintain altitude and get safely to your destination.

Alternately you can use the same trick to get to your desired altitude, and once you get there you can have an unseen servant or two (or twelve) constantly moving the pebble into and out of the bag of holding. This will give you a net vertical acceleration of zero and you can maintain aloft.

You'll still need some way to gain horizontal acceleration. Maybe a very liberal interpretation of the Wall of Force spell?

[ben-zayb]

Am I missing something with Norren's suggestion, or does the water ballast only aid in descending and maintaining altitude (adding downwards force or maibtaining vertical force equilibrium). Shouldn't you make the ship weight less than the boulder to achieve lift, and then adjust the Enveloping Ballast to have some water left over for maintaining altitude?

Also, shouldn't you apply the same horizontal force to both the ship and the boulder? The same issue of needing a sail would also apply on the boulder too, so that must be taken into consideration. Unless, of course, if the plan is to make some sort of amusement ride.

[Jack_Simth]

Am I missing something with Norren's suggestion, or does the water ballast only aid in descending and maintaining altitude (adding downwards force or maibtaining vertical force equilibrium). Shouldn't you make the ship weight less than the boulder to achieve lift, and then adjust the Enveloping Ballast to have some water left over for maintaining altitude?

Also, shouldn't you apply the same horizontal force to both the ship and the boulder? The same issue of needing a sail would also apply on the boulder too, so that must be taken into consideration. Unless, of course, if the plan is to make some sort of amusement ride.

It should, yes.

Heh.

I'm picturing two ships: One upside down in a reverse-gravity field, the other right side up, connected to each other by a couple of main masts, which have ladders on them. Both ships are constructed identically. To lift, you transfer a few crew members to the top boat. To settle, you transfer a few crew to the lower boat. When you're in port loading up, you make sure to put half of the cargo on each ship.

[Chronos]

Am I missing something with Norren's suggestion, or does the water ballast only aid in descending and maintaining altitude (adding downwards force or maibtaining vertical force equilibrium). Shouldn't you make the ship weight less than the boulder to achieve lift, and then adjust the Enveloping Ballast to have some water left over for maintaining altitude?
How is that different from what Norren suggested?

And we really shouldn't use a useless boulder, in any event. It just needs to be a weight, but there's no reason that weight couldn't be cargo, too. Just make sure that it's not cargo that'll get dropsick from being put through abrupt gravity transitions, or vertigo from having a whole empty sky "beneath" them (so, probably not passengers).

[ben-zayb]

How is that different from what Norren suggested?

And we really shouldn't use a useless boulder, in any event. It just needs to be a weight, but there's no reason that weight couldn't be cargo, too. Just make sure that it's not cargo that'll get dropsick from being put through abrupt gravity transitions, or vertigo from having a whole empty sky "beneath" them (so, probably not passengers).

Norren's addition is fine. I'm just asking if the premise (ship weighs 1000lbs) should hold. As it is, Norren's idea can't achieve vertical lift. If we reduce the initial (no water) weight of the ship, then it should work perfectly. Of course, the more we reduce the initial weight, the faster the lift would be.

[ericgrau]

Hypothetically, say you could permanently apply the effects of Reverse Gravity centered on a single 1000 lb boulder (through research into the Permanency spell, a custom magic item, whatever). The boulder now "falls" upward. We have the boulder chained to a ship, and the ship, its contents, and the chains also weight exactly 1000 lbs. Will the ship achieve "neutral buoyancy" in the air? How hard would it be to move the craft forward, assuming we can adjust the weight of the ship with ballast?

Basically yes it works. It is a blimp, with a small boulder in place of a large balloon.

Since the boulder is far smaller than a blimp there is far less air resistance and it would be pretty easy to move forward with propulsion. It would have about the same top speed as an airplane of similar size and with similar propulsion. However it would take twice as long to accelerate up to that speed because you have to accelerate twice as much mass: both the aircraft and the boulder. Not that jets have very fast acceleration to begin with. There are some super cars that can out-accelerate a jet on the runway even though they can't even come close to its top speed.

Since top speed is a bigger issue than acceleration for aircraft I wouldn't bother trying to fit something useful in place of the boulder. Not unless it's equally compact, and boulders are very compact. A boulder weighing the same as the ship will be far fewer feet across and appear tiny by comparison. Though if you really want to maximize acceleration you could make half of the ship in the reverse gravity field and use half the ship to balance the other half the ship instead of using a boulder. Cargo could work with enough ballast for the return trip.

[gadren]

So, perhaps a better design would employ a magic item that projects Reverse Gravity continually in a set area, with half the ship in the RG field, and half out, with an "anti-ballast" section actually in the RG field, working in a reverse capacity (providing more lift to counterbalance the weight of the lower section) in addition to normal ballast.

I've drawn a very crude quick sketch in MS Paint:
https://images.plurk.com/1Hr4eNBQhjNLxMGNiRsk9F.jpg

[Belial_the_Leveler]

Guys, why complicate things?


1) Ring Gates. 40.000 gp cost.
2) Place both gates side by side, facing the same way.
3) Thread wire through gates so it can be turned around extraspatially.
4) Tie one end of wire on ship, the other on winch which is also on ship.
5) Pull ship up via winch.

GGG..........GGG
..I..............I..
..I.............W..
SSSSSSSSSSS
..SSSSSSSSS..
.....SSSSS......



Essentially, you hung the ship by itself. You can hung as much weight as your wire/winch will hold. In addition, this can be used as a propulsion system. Mount the ring-gate-winch contraption on a sturdy frame, then mount the frame on a wheel. It will provide impulse towards whatever the wheel is turned towards. Need more impulse? Tighten the wire with the winch.



Mount the contraption on a medium-sized frame and cast the animate objects spell and you got a self-guiding cruise missile that can accelerate at over 10 gs with a good wire and frame, over 100 gs with hardened adamantine one. It will achieve ~5% of lightspeed within the spell's duration and will be capable of a major kinetic strike anywhere on the planet or even the moon.

[ericgrau]

I think the ring gates would fall with the ship without providing any horizontal force.

[Invader]

If you're going to use custom spell rules to make reverse gravity work the way you want why don't you you just research a fly spell that will work on objects. It makes everything loads easier and you don't have a giant bolder of doom hanging over top of your airship.

[avr]

So, perhaps a better design would employ a magic item that projects Reverse Gravity continually in a set area, with half the ship in the RG field, and half out, with an "anti-ballast" section actually in the RG field, working in a reverse capacity (providing more lift to counterbalance the weight of the lower section) in addition to normal ballast.

I've drawn a very crude quick sketch in MS Paint:

You'd get some fascinating turbulence around the edges of the field, unless it was restricted to the ship's interior. Low pressure about the bottom of the zone (the airship's midline) probably with mist and a long contrail, heating and violent turbulence about the top.

The part of the ship closest to the bottom of the zone is under a fair bit of stress as it's carrying effectively a three-storey building suspended from it; no portholes nearby, I suggest.

The ships material mostly needs to be strong in tensile strength. I.e., make it out of metal if possible.

[goto124]

If you're going to use custom spell rules to make reverse gravity work the way you want why don't you you just research a fly spell that will work on objects. It makes everything loads easier and you don't have a giant bolder of doom hanging over top of your airship.

The enemy uses that airship, and the PCs are supposed to do something with the Boulder of Doom to bring down the airship!

[nedz]

OK, assuming that the boulder is larger that 10'x10'x10' — what would happen if you simply Disintegrated a portion of it ?

[Jack_Simth]

OK, assuming that the boulder is larger that 10'x10'x10' — what would happen if you simply Disintegrated a portion of it ?
With suddenly reduced lift, the ship falls at a speed dictated by the new relative weight and whatever forms of drag apply. You can actually simulate this to a degree with a pully or two, string, and two weights. If the two weights are equal, then nothing moves. If one weight is lesser, then the heavier weight accelerates downwards while the lighter weight accelerates upwards. The pulleys exist simply to simulate the effects of the reverse gravity field.

[LooseCannoneer]

I'm not the best to comment on how the ship will move, other than that the ship is doomed when the 1 round/level duration of reverse gravity ends, and a boulder weighing as much as the ship does falls in the center of the ship.

[Jack_Simth]

I'm not the best to comment on how the ship will move, other than that the ship is doomed when the 1 round/level duration of reverse gravity ends, and a boulder weighing as much as the ship does falls in the center of the ship.
The OP mentioned that the Reverse Gravity spell is effectively permanent for the purposes of discussion.

[Coldsturm]

How big do you think this 1000lb boulder is? In the scale of things it is absolutely tiny. 6.06 cubic feet of quartz. 2x2x1.5. You could hide that anywhere, They would need to get on the ship itself to disintegrate it. I would make the "boulder" out of lead. 710 lb/ft^3 . even easier to hide and you could have a nice size ship with that counterweight.
(19,170 lb/cu yd)

[nedz]

It sounds like we can have several boulders then, cost permitting, and simply embed them within the deck. This would also stop people targeting them directly. Small lead ingots or plates would seem best.

I'm imagining now some adventurers coming across a spare one in a treasure chest somewhere.

[Baroknik]

As far as targeting a boulder with the effect -- have a spellsword channel-punch the Boulder. Bada-bing bada-boom

Edit: do objects in quintessence still exert their weight? If so just have the Boulder punched into a vat of that to maintain the spell duration

[LooseCannoneer]

Edit: do objects in quintessence still exert their weight? If so just have the Boulder punched into a vat of that to maintain the spell duration

It wouldn't make sense for them not to. Otherwise, encasing an object in quintessence would cause it to float off, making the concept of quintessence useless.

[gadren]

So would an object with a spell cast on it, encased in quintessence continue to be under the effects of that spell?

[Jack_Simth]

It sounds like we can have several boulders then, cost permitting, and simply embed them within the deck. This would also stop people targeting them directly. Small lead ingots or plates would seem best.

I'm imagining now some adventurers coming across a spare one in a treasure chest somewhere.
Yep. A large number of small(ish) weights, all independently hung (or even just resting on the ceiling in the RG area) inside a long segment of the deck could work out fairly well. At one ten-foot cube per two levels, and a 7th level spell, set it at Caster level 14 (so as to have optimal area for your GP / XP), you get 7 10-foot cubes to play with - keep line-of-effect rules intact (so it doesn't impact above-deck), and your mostly mundane ship can be kept in the air without too much hassle. With, say, a 20x40 area, you've got a 10x10 "loading section" where one worker who is rightside up can hand stuff to a worker who is upside down, and you can just move cargo and weights around as you need with standard muscle power. Ballast is a large number of bricks that are individually a heavy load for your standard worker (100 pounds for a strength-10 human). You ascend or descend by way of a hole into the reverse gravity area, in which sits a heavy metal rod, tied to a rope and pulley arrangement (you can have more than one). To lift, you crank the rod further down into the RG area. To fall, you crank it up out of the RG area (it will naturally "settle" at an effective 0 weight half in and half-out of the RG field).

You'll still need some form of thrust, of course. Lots of ways to do that, both mundane and magical.
So would an object with a spell cast on it, encased in quintessence continue to be under the effects of that spell?Like a lot of things involving Quintessence, poorly defined in RAW.

[Baroknik]

So would an object with a spell cast on it, encased in quintessence continue to be under the effects of that spell?

Like JackSmith said, the ruling is a bit nebulous.
I would think it would be acceptable to have the spell effect affect the object itself (and is possible to target it via a spellsword 2). Allowing items to still produce auras and have effects outside of he object may cause some balance issues, though.

I would personally rule of cool it -- it makes the anti-ballast easy to make/manipulate, but also makes it easy to sabotage -- just scrape some off!

[nedz]

I would personally rule of cool it -- it makes the anti-ballast easy to make/manipulate, but also makes it easy to sabotage -- just scrape some off!

Why isn't there an OCD cleaner Flaw/Trait ?

That would be hilarious:
"Why are we loosing altitude ?"
"Because someone hired a cleaner"

[Chronos]

Note that Reverse Gravity is already a valid choice for the Permanency spell. The part that needs to be houseruled/researched isn't making it permanent; it's allowing it to be cast on an object instead of a location.

[gadren]

Note that Reverse Gravity is already a valid choice for the Permanency spell. The part that needs to be houseruled/researched isn't making it permanent; it's allowing it to be cast on an object instead of a location.

Reverse Gravity is not listed as a valid choice for the Permanency spell.

[Alent]

It's been pointed out, but I managed a mistake of omission: I was making the assumption that you'd be running less than equal weight on the ship vs lift compartment, using the extradimensional storage to reduce the lift to neutral buoyancy, rather than running at 100% lift all the time.

I was picturing how the newer Aeroscraft airships handle their lift- they use pumps to compress helium cells into tanks to reduce lift.

On Quintessence based lift, I think that would be a self-resolving schrodinger's cat scenario. The object in the quintessence is falling both ways at once, but because the only observable gravity is normal gravity, the object encased in quintessence would fall until the quintessence is cracked open and the object becomes observable again.

[Chronos]

Quoth gadren:

Reverse Gravity is not listed as a valid choice for the Permanency spell.
...Huh.

Dangit, I hate it when gnomes sneak into my apartment and edit the text of my books while I'm not looking! It was definitely on the list before those pesky gnomes came along.

[Baroknik]

Note that Reverse Gravity is already a valid choice for the Permanency spell. The part that needs to be houseruled/researched isn't making it permanent; it's allowing it to be cast on an object instead of a location.

I already gave a workaround for changing the target from an area to a "target" via the Spellsword's Channel Spell ability. Just attack (and hit) a rock for non-lethal damage.

[The Random NPC]

I already gave a workaround for changing the target from an area to a "target" via the Spellsword's Channel Spell ability. Just attack (and hit) a rock for non-lethal damage.

Aren't objects immune to non-lethal damage?

[Jack_Simth]

Aren't objects immune to non-lethal damage?

Yes, but that doesn't matter. The Spellsword is just delivering a hit, it doesn't matter that the target is immune to the damage itself.

[The Random NPC]

Yes, but that doesn't matter. The Spellsword is just delivering a hit, it doesn't matter that the target is immune to the damage itself.

I kind of remember there being a general rule that states if you do no damage, rider's don't get applied. Could be misremembering though.

[Belial_the_Leveler]

I think the ring gates would fall with the ship without providing any horizontal force.

The ring gates don't provide any lift or force. They are simply used to turn the wire/rope/whatever around. Once that is done one end of the wire pulls the ship upwards, while the other end of the wire provides the counterforce... which also pulls the ship upwards. You're effectively using a portal to turn the "equal and opposite reaction" rule on its head.


It's like the story of Baron Münchhausen who pulls himself out of a river by pulling on his own hair.

[Chronos]

A pulley turns the rope around, too. I think the Ring Gates would just act like an ideal pulley.

[atemu1234]

...Huh.

Dangit, I hate it when gnomes sneak into my apartment and edit the text of my books while I'm not looking! It was definitely on the list before those pesky gnomes came along.

I know that feeling. Who made Iron Will an Ur-Priest requirement, again?

[Belial_the_Leveler]

A pulley turns the rope around, too. I think the Ring Gates would just act like an ideal pulley.

The difference is that the rope exerts force on the pulley while it doesn't on the gates. With a pulley, even an ideal one, you can't break the balance of forces; with the gates you can.

[Chronos]

The difference is that the rope exerts force on the pulley while it doesn't on the gates.
Are you sure about that? The rules never say they do, but they also never say they don't, leaving us to rely on our own common sense to decide which is the case... except, ring gates being absent in our world, nobody's ever had a chance to develop common sense about them, which means people can come to different conclusions about how it works. To you, it's common sense that the gates would not exert a force on the ropes, but to ericgrau and I, it's common sense that they would, and the rules don't really give enough information to decide whose common sense is right or wrong.

[Belial_the_Leveler]

The rope is not touching the gates. How in the name of Newton is it going to exert force on them? If it would still somehow exert force, you could always go for incorporeal rope tied to ghost-touched winch. Thus by the game rules, it can't exert force on the gates at all.

[gadren]

The rope is not touching the gates. How in the name of Newton is it going to exert force on them? If it would still somehow exert force, you could always go for incorporeal rope tied to ghost-touched winch. Thus by the game rules, it can't exert force on the gates at all.

Perhaps you could draw a diagram? I admit I'm having a hard time visualizing this idea.

[CGNefarious]

If you're using gravity to supply and manage the lift, why not use it to supply the horizontal force as well? Based on this conversation I'm assuming ring gates work similar to the holes in Portal? Create a few slopes toward the back of the ship that angle in different directions. Put one gate at the top of the slope facing downward and the other at the bottom of the slope facing upwards. Drop rocks (or other heavy rolling objects) into the bottom gate. They will then come out of the top gate and slide down the ramp, countering the normal force of the ramp and turning the completely vertical force of gravity into both vertical and horizontal components. Then when they reach the bottom of the ramp they will enter the original ring gate, repeating the process and essentially creating and infinitely long ramp for them to roll down. The horizontal component should be able to drive the ship in the opposite direction of the ramp and the vertical component can be countered by the other many methods already talked about. Now you can have a completely mundane, albeit intelligent, crew drive the ship. You'd still need a decent amount of time to decelerate, but it's a pretty simple way to power the ship.

For quick stops I think your best solution is going to be finding a way to create a lot of air resistance. Maybe very large sails that you only open up when you want to slow down. Or a parachute of some kind.

If you use my previous idea about the slopes and ring gates then you can use those to create a force in the opposite direction travel to decelerate as well.

Unless what I said makes absolutely no sense to anybody...

[The Random NPC]

Ring gates have a daily limit of 100 pounds.

[Baroknik]

For propulsion I always like using Decanters of Endless water. Just strap 'em to the back of the ship and say the command words! It's nice that you can have a stepped acceleration too by just opening a fraction at a time.

[Jack_Simth]

Ring gates have a daily limit of 100 pounds.
However, an object that's partly pushed through and then retracted doesn't count. If you seal both ends of the rope, it never finishes going through. Doesn't work with boulders, though. Chain maybe, and could be used to a similar effect.

[Belial_the_Leveler]

Ring gates have a daily limit of 100 pounds.

100 pounds of wire 5 feet long is a three-inch-thick steel bridge cable that could lift dozens of tons.
Also, objects partially extended through the gates and then retracted do not count vs the daily limit.

[The Random NPC]

However, an object that's partly pushed through and then retracted doesn't count. If you seal both ends of the rope, it never finishes going through. Doesn't work with boulders, though. Chain maybe, and could be used to a similar effect.

I'm aware of the partial thing, I was mostly replying to CGNefarious.


100 pounds of wire 5 feet long is a three-inch-thick steel bridge cable that could lift dozens of tons.
Also, objects partially extended through the gates and then retracted do not count vs the daily limit.

I did think of something, there has to be some force on the ring gates, or they would just fall to the ground. I think the wires would be the support, but then they'd have to be more rigid. Also I think you'd only get half of the cable's length in height.

[Jack_Simth]

Hmm... what would a cross-section look like... two basic options...

Brown is the material of the ship (or mounted pulleys, where it's obvious), black is the rope/chain/whatever, gold is the ring gates (I'm assuming the ring gates themselves are glued to the ship, the rope is left free to move).

http://s23.postimg.org/sx8phqsav/ring_gate_rough.jpg (http://postimg.org/image/sx8phqsav/)

On the left, we have the infinite strand (can be arranged up, down, left, right, whatever). You tug it into the gate, one way or the other, to exert force on yourself, which translates through your body into the ship. Problem: It's no more efficient than throwing stuff out the back of the ship in terms of acceleration, other than that you never lose the mass. You could drop it, let gravity do it's thing, and then grab onto it to get a little force pushing you downwards... but it's not really going to do much.

On the right, we have the anti-pully. If you hang anything from that rope, it goes nowhere. Force of tugging applies on the pulleys, but not the ship wall (there's no method by which the rope can affect the ship wall or the ring gates). Depending on the specific alignment of the device, if you place tension on the rope, then you should get some sourceless thrust (put tension on rope: Rope pulls the two pulleys towards each other: These are tied to the ship, and the up/down force cancels. The rope pulls on itself through the ring-gate - and due to the nature of the ring-gate, the force there is countered by the rope itself; rope also pulls on the two pulleys, and is pulling both of the two pullies to the right... no other force is applied to the ship from the arrangement, so the ship accelerates to the right). However: The use of this assumes that the rope cannot exert any force on the ring gates. If this is true, then you can break any such rope with a pair of ring gates: Simply move the two ring gates further apart, and infinite tension is applied to the rope. If this is not true, then the contraption does not work. Thrust is limited to the strength of your materials and how much tension you can put on the rope.

[dascarletm]

The enemy uses that airship, and the PCs are supposed to do something with the Boulder of Doom to bring down the airship!

I can see it now...

"Cut the ropes attached to the boulder!"
"Look out Stephan! That bit of rope is tangled around your leg!"
"What? Oh NOOOOOoo...
"Stephan!"

[Jack_Simth]

Hmm... eh, it's still February....

Continuing on my image from earlier, with the two pulleys and the ring gate pair (assuming best interpretation):

The effective force pushing on your ship from the contraption is applied to your ship at the pulleys - so make sure those are solid.
The effective force pushing on your ship is equal to twice the tension on the rope (and is distributed essentially evenly between the two pulleys... although those pulleys are taking more total force than the tension on the rope, because the rope pulls them towards each other exactly as hard as it pulls them towards the ring gates).
How you put tension on the rope is almost completely irrelevant. A lever pushing another pulley in line between the two mains to stretch the rope towards the ring gates would work well, as it lets you control how much force you're applying quite well.

[gadren]

Hmm... what would a cross-section look like... two basic options...

Brown is the material of the ship (or mounted pulleys, where it's obvious), black is the rope/chain/whatever, gold is the ring gates (I'm assuming the ring gates themselves are glued to the ship, the rope is left free to move).

http://s23.postimg.org/sx8phqsav/ring_gate_rough.jpg (http://postimg.org/image/sx8phqsav/)

On the left, we have the infinite strand (can be arranged up, down, left, right, whatever). You tug it into the gate, one way or the other, to exert force on yourself, which translates through your body into the ship. Problem: It's no more efficient than throwing stuff out the back of the ship in terms of acceleration, other than that you never lose the mass. You could drop it, let gravity do it's thing, and then grab onto it to get a little force pushing you downwards... but it's not really going to do much.

On the right, we have the anti-pully. If you hang anything from that rope, it goes nowhere. Force of tugging applies on the pulleys, but not the ship wall (there's no method by which the rope can affect the ship wall or the ring gates). Depending on the specific alignment of the device, if you place tension on the rope, then you should get some sourceless thrust (put tension on rope: Rope pulls the two pulleys towards each other: These are tied to the ship, and the up/down force cancels. The rope pulls on itself through the ring-gate - and due to the nature of the ring-gate, the force there is countered by the rope itself; rope also pulls on the two pulleys, and is pulling both of the two pullies to the right... no other force is applied to the ship from the arrangement, so the ship accelerates to the right). However: The use of this assumes that the rope cannot exert any force on the ring gates. If this is true, then you can break any such rope with a pair of ring gates: Simply move the two ring gates further apart, and infinite tension is applied to the rope. If this is not true, then the contraption does not work. Thrust is limited to the strength of your materials and how much tension you can put on the rope.

I'm sorry, but neither of those diagrams make sense to me.

[Jack_Simth]

I'm not very good at drawing, sorry.

Suppose you want to apply force to go forwards (should be a pretty common task on a ship, no?).

You affix a pair of ring gates to the inside of the ship, with the non-passing "back" of the ring gates on the inside wall facing the direction you will want to travel (the bow), and the matter-passing "front" of the ring gates pointing towards the back of the ship (the stern).

You thread a rope through the gates and around two pillars built into the ship for the purpose. You then tie the ends of the rope together.

To make the ship go forward, you apply tension on the rope (What method you use to apply tension doesn't matter).

A rope cannot push; it can only pull. It pulls on everything evenly. However: The ring gates reverse the direction. At that end, the rope is pulling only on itself, not any part of the ship (the rope does not apply force to the ring gates per assumptions - this would be testable if we actually had ring gates, but eh). The rope pulls both pillars in towards each other (inwards force on any given pillar = the tension on the rope). The rope pulls both pillars towards the ring gates (forwards force on each pillar = tension on the rope). The ring gates apply the tension on the rope against itself and nothing else, and thus there is no force pulling the wall of the ship back towards the pillars.

This leaves you with a force imbalance on the ship (equal to twice the tension on the rope, one iteration of the tension on the rope per pillar), and when force is not balanced, you get acceleration according to A=F/M (Acceleration = Force / Mass). Frictive forces such as wind resistance will put a limit on your speed (wind resistance goes up with velocity, so you'll stop accelerating when the force of wind resistance equals twice the tension on your rope), but otherwise your ultimate speed limit is going to be the absolute speed limit (the speed of light is thought to be that in reality).

The strength of your materials is going to limit how much force you can apply (there is some tension at which either the pillars, the mounting on the pillars, or the rope will give out), as is the amount of tension you can arrange to put on the rope. Well, also how good you are at knots that won't come undone under strain.

You can add more pillars, but unless you also add more ring gates, they don't have any extra effect. Each pair of ring gates, and each pair of pillars, with the same threaded rope, you get twice the tension on the rope as thrust on your ship.

Make sense?

[The Random NPC]

...

Wouldn't whatever you're using to apply tension to the ropes provide the counterbalancing force?

[SangoProduction]

I've only read the first page, so this might have already been stated but...

Assuming you could just affect a single object with the spell, and that it inverses the effects of gravity on it. For simplicity's sake, we'll assume Newtonian gravity rather than relativity-based gravity. (because trying to visualize relativity's gravity in 3d is hard...trying to picture the inverse of that ship is absurd) We will also assume the mass of both objects is = to some number. We will also assume whatever tether you have is also infinitely sturdy. Negative gravity also implies upward motion. We will also ignore any objects not mentioned in the example.

What would normally happen is that these 2 objects would pull together. With the inverse of this being that they push away. So, tied together, they would not move because they both want to pull with the same force in opposite directions, negating themselves - regardless of their mass relative to eachother.

BUT! We are forgetting that gravity works on all objects, not just 2 at a time. So, the planet is exerting it's gravity. When inversed, it pulls the rock up. However, as you move further away from a source of gravity, the gravity becomes less powerful. So, the rock, above the ship, might have an effective gravity of -90%, but the ship below might have an effective gravity of 95%, netting a 5% gravity difference, because it's closer to the planet. Thus, it pulls the ship downwards. (the real difference would likely be smaller, but it will always be there)

To be fair though, this does effectively mean it weighs less than it would normally, but on it's own, it doesn't help.

This problem isn't solved by putting the anti-gravity thing under the ship either, as it will try to push away from the ship, pulling it downwards. However, it will also want to push away from the planet more, because that has more gravity, pushing it upwards more than the ship is pulling downwards.

So, it would accelerate upwards....and keep accelerating, though it's acceleration would admittedly slow, it wouldn't lose it's momentum it's already gained except through friction, or an outside force trying to keep it down, and even then, you'd still be accelerating away. Even the international space station still experiences 90% of the gravity that you'd find on Earth (it's just falling and "missing" the planet, and thus orbits).

So, unless you want to be trying to fly downwards so you stay stable, or you want to experience the outer reaches of the solar system, this would not really be an option either.

If we were to say that the object that was on top actually had more mass (to compensate for the distance difference in gravity), then it would be conceivable to strike a balance between the ship's downward pull, and the rock's upward pull.

As mentioned previously, I am not accounting for any moons/birds/or space stations for simplicity's sake.

[Jack_Simth]

Wouldn't whatever you're using to apply tension to the ropes provide the counterbalancing force?

Nope. There's a rather lot of ways to apply tension to the rope.

If you want to make it pretty clear that there's no need for contact:
You can use a Ratchet style tie-down stage (http://www.uline.com/Product/Detail/H-2740/Dock-Equipment/Flat-Hook-Ratchet-Style-Tie-Downs-2-x-12-10000-lb-capacity?pricode=WY575&gadtype=pla&id=H-2740&gclid=CN7kjf7Vg8QCFY17fgodMhMATg&gclsrc=aw.ds) or similar.
You can hang a weight off a horizontal section of the rope (doesn't matter where in line). When gravity pulls on the weight, it forces the angle to change on the rope. The rope can only pull, and in order to reach equilibrium (stillness) the tension of hanging a small weight off of a rope that starts out even slightly taught has some surprisingly large force-multiplication effects (e.g., with the right setup, it's quite straightforward to put, say, 100 pounds of tension on the line with just a one pound weight). See Here (http://demonstrations.wolfram.com/TensionOfARopeWithAHangingMass/) for the equations if you want.

However: Even if you do full contact: It doesn't matter. Suppose you're standing between the two pillars, and push the rope towards the ring gates. Any force you apply to the rope by pushing on it adds tension to the rope the same way a weight hanging down would. The force the rope applies that would stop the weight from falling is applied back to you (and is exactly equal to the force you're applying to the rope, once the system reaches equilibrium). That force is also applied to the deck. However: It's ALSO applied to the pillars. The tension remains on the rope (and thus, the thrust from the system due to the physics cheat that is the ring gates) however, all of the forces you're applying beyond that tension end up negating each other exactly. That can be a guy pushing on the rope. That can be a guy standing on the top deck pushing on a lever that pushes on the rope. Doesn't matter, the math works out the same way.

[The Random NPC]

Nope. There's a rather lot of ways to apply tension to the rope.

If you want to make it pretty clear that there's no need for contact:
You can use a Ratchet style tie-down stage (http://www.uline.com/Product/Detail/H-2740/Dock-Equipment/Flat-Hook-Ratchet-Style-Tie-Downs-2-x-12-10000-lb-capacity?pricode=WY575&gadtype=pla&id=H-2740&gclid=CN7kjf7Vg8QCFY17fgodMhMATg&gclsrc=aw.ds) or similar.
You can hang a weight off a horizontal section of the rope (doesn't matter where in line). When gravity pulls on the weight, it forces the angle to change on the rope. The rope can only pull, and in order to reach equilibrium (stillness) the tension of hanging a small weight off of a rope that starts out even slightly taught has some surprisingly large force-multiplication effects (e.g., with the right setup, it's quite straightforward to put, say, 100 pounds of tension on the line with just a one pound weight). See Here (http://demonstrations.wolfram.com/TensionOfARopeWithAHangingMass/) for the equations if you want.

However: Even if you do full contact: It doesn't matter. Suppose you're standing between the two pillars, and push the rope towards the ring gates. Any force you apply to the rope by pushing on it adds tension to the rope the same way a weight hanging down would. The force the rope applies that would stop the weight from falling is applied back to you (and is exactly equal to the force you're applying to the rope, once the system reaches equilibrium). That force is also applied to the deck. However: It's ALSO applied to the pillars. The tension remains on the rope (and thus, the thrust from the system due to the physics cheat that is the ring gates) however, all of the forces you're applying beyond that tension end up negating each other exactly. That can be a guy pushing on the rope. That can be a guy standing on the top deck pushing on a lever that pushes on the rope. Doesn't matter, the math works out the same way.

Huh, seems to work out, trippy.

[Chronos]

I still say the rope exerts a force on the ring gates. What happens if you just have the gates facing each other directly with the loop of rope between them, and shorten the rope? What happens if you try to move the rings further apart? I can't see any answer that works, unless there's a coupling between the rope and the portals.

[Jack_Simth]

I still say the rope exerts a force on the ring gates. What happens if you just have the gates facing each other directly with the loop of rope between them, and shorten the rope? What happens if you try to move the rings further apart? I can't see any answer that works, unless there's a coupling between the rope and the portals.
I mentioned that assumption on my post with the diagram:

However: The use of this assumes that the rope cannot exert any force on the ring gates. If this is true, then you can break any such rope with a pair of ring gates: Simply move the two ring gates further apart, and infinite tension is applied to the rope. If this is not true, then the contraption does not work.
I've also made reference to it a couple of times.

The trouble is that whether or not there's any coupling on the gates is not clearly specified one way or the other. Hence why I called out non-coupling as an assumption. We can make some guesses, though. Let's take a look at The Ring Gate Description (http://www.d20srd.org/srd/magicItems/wondrousItems.htm#ringGates)
Ring Gates

These always come in pairs—two iron rings, each about 18 inches in diameter. The rings must be on the same plane of existence and within 100 miles of each other to function. Whatever is put through one ring comes out the other, and up to 100 pounds of material can be transferred each day. (Objects only partially pushed through and then retracted do not count.) This useful device allows for instantaneous transport of items or messages, and even attacks. A character can reach through to grab things near the other ring, or even stab a weapon through if so desired. Alternatively, a character could stick his head through to look around. A spellcaster could even cast a spell through a ring gate. A Small character can make a DC 13 Escape Artist check to slip through. Creatures of Tiny, Diminutive, or Fine size can pass through easily. Each ring has a "entry side" and an "exit side," both marked with appropriate symbols.

Strong conjuration; CL 17th; Craft Wondrous Item, gate; Price 40,000 gp; Weight 1 lb. each.

- Specifically, we want to know what sorts of things you can do with them: Transport items, messages, attacks; grab stuff near the ring, stab a weapon, look around, cast a spell, slip through if you're small or smaller.

"Stab a weapon" however, mostly implies that there's little, if any, coupling at the Ring Gate. If there were, it would seriously reduce the effectiveness of any weapon used through the gate.

However, it's not clearly stated, so it's up to the individual DM on what can, and can not, be done via a Ring Gate.

[dascarletm]

I've only read the first page, so this might have already been stated but...

Assuming you could just affect a single object with the spell, and that it inverses the effects of gravity on it. For simplicity's sake, we'll assume Newtonian gravity rather than relativity-based gravity. (because trying to visualize relativity's gravity in 3d is hard...trying to picture the inverse of that ship is absurd) We will also assume the mass of both objects is = to some number. We will also assume whatever tether you have is also infinitely sturdy. Negative gravity also implies upward motion. We will also ignore any objects not mentioned in the example.

What would normally happen is that these 2 objects would pull together. With the inverse of this being that they push away. So, tied together, they would not move because they both want to pull with the same force in opposite directions, negating themselves - regardless of their mass relative to eachother.

BUT! We are forgetting that gravity works on all objects, not just 2 at a time. So, the planet is exerting it's gravity. When inversed, it pulls the rock up. However, as you move further away from a source of gravity, the gravity becomes less powerful. So, the rock, above the ship, might have an effective gravity of -90%, but the ship below might have an effective gravity of 95%, netting a 5% gravity difference, because it's closer to the planet. Thus, it pulls the ship downwards. (the real difference would likely be smaller, but it will always be there)

To be fair though, this does effectively mean it weighs less than it would normally, but on it's own, it doesn't help.

This problem isn't solved by putting the anti-gravity thing under the ship either, as it will try to push away from the ship, pulling it downwards. However, it will also want to push away from the planet more, because that has more gravity, pushing it upwards more than the ship is pulling downwards.

So, it would accelerate upwards....and keep accelerating, though it's acceleration would admittedly slow, it wouldn't lose it's momentum it's already gained except through friction, or an outside force trying to keep it down, and even then, you'd still be accelerating away. Even the international space station still experiences 90% of the gravity that you'd find on Earth (it's just falling and "missing" the planet, and thus orbits).

So, unless you want to be trying to fly downwards so you stay stable, or you want to experience the outer reaches of the solar system, this would not really be an option either.

If we were to say that the object that was on top actually had more mass (to compensate for the distance difference in gravity), then it would be conceivable to strike a balance between the ship's downward pull, and the rock's upward pull.

As mentioned previously, I am not accounting for any moons/birds/or space stations for simplicity's sake.

The difference in gravity from the ship and the rock would be so negligible that they would in effect be equivalent.

F= G(m1m2/r2)

Δr=roughly I don't know 10 meters?
Mass of boulder/ship 1000kg
Mass of planet (assuming like earth) 5.972x1024kg
Gravitational Constant 6.673×10−11 N·(m/kg)2

Let's say we are flying 6,375 km from the the planet's core.

Force applied to ship: 9805.74N
Force applied to boulder: 9805.71N

pardon if I made an error.