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

[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).



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 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 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 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 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

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.

Spoiler: Math

Show

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.