PDA

View Full Version : Liberal arts major DM needs some physics advice.



Blad
2013-01-03, 08:54 AM
Next session, I plan on introducing the PCs to a treasure trove of something called ‘moonstones’.

The properties of moonstones are thus that you can’t store too many of them in one place, so there’ll be an underground vault with rows upon rows of heavy metal chests, each holding a single rock or a couple of small ones.

If the chests are opened, the moonstones drift up to the ceiling and stay there. This is because the defining characteristic of moonstones is that they are really, really light. Lighter than air.

Now, bearing in mind that I’m not shooting for strict realism but fun and verisimilitude,

1. Would it be okay to simply treat the moonstones as things that (on the surface of the earth) have ‘negative weight’?

As in, a chest that weighs 20 pounds;
- effectively weighs only 5 pounds if you put in -15 pounds’ worth of moonstones,
- is effectively weightless if you put in -20 pounds worth,
- slowly drifts up if you add -25 pounds,
- shoots, or ‘falls’ upward, into the sky the moment you let go of it if you put in -150 pounds, and
- falls upward into the sky taking you with it if the negative weight of the moonstones is more than the weight of the chest and yourself combined.

2. If so, would this also mean that;

- lifting a 50 pound object off the ground requires the same amount of strength as forcing a -50 pound object down onto the ground?
- preventing a 50 pound object from falling to the ground requires the same amount of strength as preventing a -50 pound object from falling into the sky?

3. If an object that, on the surface of the earth, weighs -50 pounds is released, how far will it continue to fall/float up? And if it weighs -100 or -1,000 pounds?

4. To what use, other than novelty jewelry, could you put the moonstones? I’ve so far come up with the following ideas; would they work?

- A personal ‘gravity-inhibitor’ that you carry on your body and that grants you a bonus to (some) athletics, acrobatics, endurance and/or stealth checks.
(if so, what would be a good amount to use, what kind of bonus is suitable for that amount, and do you think it should require specially prepared boots or otherwise take up an item slot?)
- A flotation device for sailors (one that you have to carry on your person at all times, because if you’re already in the water they can’t exactly throw it at you).
And on a somewhat larger scale;
- An all-purpose ‘cargo-lightener’ (while it’s properly attached);
- A building material for structures otherwise too big to be feasible in a pre-industrial setting;
- ‘Reverse piledriving’; hollowing out a cavern by taking large quantities of moonstones, letting them crash into the ceiling, forcing them back down and repeating (Yes, that’s a dangerous thing to do if you’re directly underneath. But then, most things D&D adventurers do are dangerous.)
- My personal favorite: turning a small, regular sailing ship into an airship, by putting a lot of the moonstones below deck, evenly distributed, or by gathering them into a net or a sack that’s attached to the boat, and then dumping ballast, untying the mooring lines and/or weighing anchor.

5. Apart from the fact that you always have to weigh them down to stop them from flying away (which makes you wonder how they came to exist on earth to begin with, but never mind) would moonstones have serious downsides I haven’t considered?

6. Finally, taking into account how much of what I have thought up is physically feasible, and considering they are very rare, indeed virtually unknown, how much would a negative pound of moonstone be worth to the right buyer?

Any help would be greatly appreciated.

Adoendithas
2013-01-03, 09:13 AM
1) Yes, but doing this breaks the Equivalence Principle if you're doing any calculations (they still have positive mass in the sense that you can apply force to them).

2) Yes, although one or the other might be easier depending on which muscles you're using. It would require the same amount of force.

3) It would depend on how dense the air is at ground level, and how dense the moonstones are. If they truly had negative mass they'd just keep flying upwards forever, swerving away from other planets and such.

4) Those all sound feasible and awesome, although be careful to keep them distributed evenly. If you made Boots of Springing with these you would probably end up doing a sort of backward somersault and hitting your head on the ground.

5) If they have very low inertial mass (since they're lighter than air is), they wouldn't require a lot of force to accelerate. You could throw -150 lbs much farther downwards than you could throw 150 lbs upwards, since (although gravity pulls the same on both) the negative-mass one is much lighter.

6) Probably a lot, since they sound really useful and you could lose them so easily. They might be able to exist on Earth if they weren't fully solid (hollow and filled with helium, for example, or in the form of a special aerogel), but I'm not sure about this.

By the way, do you mind if I borrow some properties of these for my campaign now? :smallredface:

Hal
2013-01-03, 09:32 AM
I think your rules on how to treat them (i.e. negative weight) are fine; probably the best approximation you're going to get in the game without resorting to cumbersome math.

The better question is, to what end are you introducing these? What do you expect the players to get out of having them? Are you going to provide enough that they can make an airship/piledriver/etc.?

What happens to these when you put them in a bag of holding (or some other extra-dimensional space)?

Also, these are the kind of thing that can kill a player if they're not aware of what they're holding, in the sense that someone might grab a big handful of them and float away. Conversely, be aware that they will try to find a way to kill an NPC the same way: Sneak enough of these into their pockets to make them fly away.

As for the cost . . . well, let's try some math!

A sailing ship costs 10,000 gold and weights 300,000 lbs. An airship should weigh similar, but costs 85,000 gold. Let's say the difference in cost is for the amount of moonstone to raise 300,000 lbs. That's 75,000 gold for 300,000 lbs., or 0.25 gold/lbs.

Is that reasonable? Well, I'm not sure. Lifting a single person will run you ~75 gold (assuming a person with a total weight of 300 lbs.) You might want to consider scaling the costs as necessary.

Blad
2013-02-01, 04:03 PM
Wow. I didn’t expect to go so long without posting. Some Stuff TM came up.
But, as the previous gaming session ended just short of the PCs getting to the treasure room, I’ve had some more time to play around with the concept and I thought I’d bring it up on the off-chance that somebody still cares. Warning, huge wall of text incoming.

In retrospect, I probably should have taken Hal’s approach from the start. D&D already has ways to make things float and an economy to go with it, so it makes more sense to come up with variations on existing stuff, rather than invent stuff from scratch first and try to mesh it with the existing system later.

However, that’s bad news for the possibility of creating your own improvisational airship (which I would really like the PCs to be able to do, in case that wasn’t already obvious). It seems if the Gods of D&D had meant for heroic-level characters to fly, they would have given them bigger treasure parcels.
Hal has demonstrated this by comparing the PHB sailing ship and the Adventurers’ Vault Airship, and that comparison doesn’t even take into account that the sailing ship, which carries 150 tons, is doubtlessly way bigger and heavier than the airship, which carries only 20 tons. Likewise, still going by the Adventurers’ Vault, the price of a regular mount is only a fraction of that of a flying mount which in all other respects is more or less the same. Even an ornithopter (the cheapest way to fly) will still cost at least 2,800 gp more than a chariot and a mount to move it.

It’s also not easy to express it in terms of ‘negative weight’, for the simple reason that none of the vehicles or mounts have their weight listed, only their carrying capacity. 150 tons is how much cargo the sailing ship can carry, not the weight of the ship itself. But to make something fly, you need to lift the thing itself as well as all it carries.

It’s even more unbalanced if the item that allows you to fly also gives you skill bonuses. By PHB standards, something that gives you a +1 to Strength and Athletics checks, one daily power, and nothing else, is already a level 5 magic item worth 1.000 gp. and it doesn’t even have anything to do with flying.

Then again that’s also part of why I like the idea of moonstones: the earlier-edition-ish feel of ‘here is a thing that behaves in strange ways, see if you can figure out more about how it works and how you can use it’, rather than ‘you receive this item, listed on this page of the PHB, exactly this powerful and valuable, and it was specifically made to do this one thing and nothing else’.

I came up with the following stats to keep moonstones manageable, low on cumbersome math and appropriately balanced within 4th edition, while hopefully also still somewhat interesting. I split them up into three kinds, according to what you can use them for. I neutered the ‘negative weight’-aspect; moonstones and items with moonstones on them don’t actually have negative weight, but in certain respects they behave as if they did.

***

Minor Moonstone
This is a grey, orb-like object that fits in the palm of your hand. It is cold and hard to the touch.
Anyone who spends a few minutes examining a minor moonstone learns the following:
This stone has some strange supernatural properties.
Firstly, it can affix itself extremely firmly to the surface of other objects and does so whenever you place it against something, but it also responds to your touch, allowing you to easily dislodge it again.
Secondly, and more importantly, this stone, in some ways, seems to suspend gravity. If you don’t hold onto it or affix it to something, it floats up into the sky. And if you do affix it to something, it transfers its properties onto the thing it’s affixed to, making it lighter. This one, for instance, seems to make objects weigh about 20 pounds less, and if you stick it onto something that already weighs less than 20 pounds, that object floats up with it. Other than that, it seems to have no effect on the object.
Anyone who compares a minor moonstone with another moonstone learns the following:
If you stick two minor moonstones onto something, it weighs 40 pounds less, and it starts floating up if it already weighs less than 40 pounds. However, like magnets, moonstones seem to repel each other. No more than two will ever stay affixed to an item at a time. Nevertheless, they could prove useful in many minor ways, if only because they enable you to carry more loot.
Anyone who, after examining a minor moonstone, succeeds on a DC 10 Arcana check, learns the following:
The moonstone is probably a hitherto unknown magical creation, or some sort of earthly manifestation of a supernatural elemental force. Either way, apart from its practical applications it should be an object of interest to any scholar of abnormal phenomena. In other words, moonstones, particularly large quantities of them, should be very valuable to the right people, if only you can find them.
Affix
A moonstone can affix itself to a surface it touches. You can affix a moonstone onto another item as a minor action or by using it as ammunition for a ranged attack (proficiency bonus 0, range 10/15, 0 damage, and the moonstone flies away if you miss). It will stay firmly affixed until you remove it as a minor action.
You can never affix a moonstone to another moonstone. You can never affix more than two moonstones to a single item; affixing a third causes the first one to detach. (A nine-by-nine squares area of floor counts as one item for this purpose.) You can store as many moonstones as you like in a container big enough to fit them, but only two will affix themselves to it, and the others will float out as soon as you open it.
Lighten
While a minor moonstone is affixed to an item, that item’s weight is reduced by 20 lbs. If you affix a minor moonstone to a container, whether on the outside or on the inside, the weight of the entire container is reduced by 20 lbs. (However, if the container is a magical one that ignores the weight of its contents, such as a bag of holding, nothing happens.)
It is possible to reduce an item’s effective weight to 0 (but never to less than 0).
You can affix a minor moonstone to a melee weapon but you take a -2 to all attack rolls and a -6 to all damage rolls with it. You can affix a minor moonstone to a piece of ammunition but that prevents you from attacking with it.
Levitate
If left unattended or unaffixed, the moonstone floats off into the sky, or against the ceiling if indoors. If it is not stopped, the moonstone ascends at a speed of 50 feet (10 squares) per round until it is 500 ft above the ground, at which point it stops moving.
If a moonstone is affixed to an item that weighs less than 20 lbs, or if two moonstones are affixed to an item that weighs less than 40 lbs, that item starts floating like a moonstone does.
An item that weighs more than 40 lbs in its natural state doesn’t float no matter how many moonstones you stick on it.

***

Major Moonstone
This looks and feels like the minor moonstone, except it’s twice as big and has a strange, otherworldly bluish color.
Anyone who spends a few minutes examining a major moonstone learns the following:
Like a minor moonstone, a major moonstone can affix itself extremely firmly to the surface of other objects and does so whenever you place it against something, but it also responds to your touch, allowing you to easily dislodge it again. However, it works a little different in that there are many items it simply will not affix itself to – typically, items that are either very small, or very big. Major moonstones also do not make the items they’re affixed to any lighter, let alone cause them to fly off, but their influence shows in a slightly different way: anything with a major moonstone affixed to it feels as though some strange energy is pulsating through it, and when moved it moves faster, further, or both.
Anyone who, after examining a minor moonstone, succeeds on an Arcana check, learns the following:
DC 10: Like a minor moonstone, the major moonstone is a strange supernatural phenomenon and probably valuable for that reason alone.
DC 15: More importantly, unlike a minor moonstone, which just makes it easier to move things, a major moonstone also affects how, how far, and how fast things move. It would be very interesting to affix this moonstone to living creatures and vehicles to produce new effects. What exactly these effects are and how many moonstones they take to achieve, you will only know if you try it out for yourself, but the ramifications could be huge.
Affix
A moonstone can affix itself to a surface it touches. You can affix a moonstone onto another item as a minor action or as a ranged attack (proficiency bonus 0, range 10/15, 0 damage, and the moonstone flies away if you miss). It will stay firmly affixed until you remove it as a minor action.
You can never affix a moonstone to another moonstone. You can never affix more than two moonstones to a single item; affixing a third causes the smallest of the three to detach. You can never affix a major moonstone to an item that weighs less than 50 lbs, or to an item that takes up more than 4 squares (including ‘the floor’ or ‘the ground’).
However, you can store as many moonstones as you like in a container big enough to fit them.
Moonstone Empowerment
While a major moonstone is affixed to (or inside) an object, everyone gets a +1 to Strength or Athletics checks to move that object.
When the object is a living creature, that creature
- gains a +1 bonus to Climb, Jump, and Swim checks;
- gains a +1 bonus to Acrobatics checks;
- when it’s subject to forced movement, moves 1 more square than it normally would.
When the object is a vehicle, its speed is increased by 1.
Double Moonstone Empowerment
While two major moonstones are affixed to (or inside) an object, everyone gets a +2 to Strength or Athletics checks to move that object, and the object never takes damage from any fall, regardless of its distance.
When the object is a living creature, that creature
- gains a +1 bonus to speed;
- gains a +2 bonus to Climb, Jump, and Swim checks;
- gains a +2 bonus to Acrobatics checks;
- has to make an immediate saving throw after it moves, or is moved, 4 squares or more as part of a single action; if it fails, the creature continues moving in the same direction for another 4 squares;
When the object is a vehicle, its speed is increased by 2. Unless it’s loaded to full capacity (in terms of pounds of cargo), the vehicle also starts to hover. Whenever not fully loaded it hovers exactly a foot above the ground (or, when over water, above the surface). It can never get higher than that, unless something else allows it to do so.
If the vehicle can still move while not touching the ground, its hover speed is the same as that of its regular movement.
While hovering, the vehicle ignores difficult terrain, but creatures pushing or pulling the vehicle don’t.
Levitate
If left unattended or unaffixed, the moonstone floats off into the sky, or against the ceiling if indoors. If it is not stopped, the moonstone ascends at a speed of 50 feet (10 squares) per round until it is 500 ft above the ground, at which point it stops moving.
It never does this when still affixed to an object.

***


Master Moonstone
It looks exactly like the minor moonstone, except around 5 feet in diameter and if you come near, you can hear it hum softly.
A huge metal chain, that must be several hundreds of feet long and looks as though it’s strong enough to carry hundreds of pounds, is attached to a large ring inserted into the moonstone.
Anyone who spends a few minutes examining the master moonstone learns the following:
This moonstone doesn’t seem to want to affix itself to objects – or at least not to most objects, and that includes you and your companions. It can affix itself to the ground pretty well, though, and in so doing it can detach other moonstones affixed to the ground. Affixing and dislodging the master moonstone is a standard action. Like the other moonstones, the master moonstone feels weightless, even if you factor in the huge iron chain. However, this property doesn’t translate to anything you attach to the stone or the chain by conventional means.
Like the other moonstones, if you don’t hold onto it or affix it to something, it floats up into the sky; there’s so much force behind it that it might even damage objects it crashes into on its way up. However, the moonstone still does what you want it to do: if you hold the stone or the chain, you can immediately stop it from floating off and you can move it along with you everywhere you go with no effort whatsoever. It’s only moving the moonstone closer to earth that requires you to make a Strength check.
Anyone who, after examining a master moonstone, succeeds on an Arcana check, learns the following:
DC 15: At first glance it might seem that the master moonstone has less practical applications than the major moonstones, which could be affixed to living creatures and small vehicles. But it might also be the case that all the objects you tried to affix the moonstone to, with the exception of ‘the ground’, were simply too small. If so, then who knows what might happen if you attached a major moonstone to something huge or gargantuan.
Affix
A master moonstone can affix itself to a surface it touches. You can affix a master moonstone onto another item as a standard action. It will stay firmly affixed until you remove it as a standard action.
You can never affix a moonstone to another moonstone. You can never affix more than two moonstones to a single item. Affixing a third causes the smallest of the first two to detach. You can never affix a master moonstone to an item that’s not huge or gargantuan in size. Affixing a master moonstone to an item causes all other moonstones affixed to that item within 4 squares of it to detach. (‘The ground’ or ‘the floor’ counts as one item.) However, you can store as many master moonstones as you like in any container big enough to hold them.
‘Affixing’ the stone to something is not the same thing as attaching it to something using the chain, or indeed interacting with the chain in any way. You can use the chain to keep the master moonstone from floating off, move it along with you, pull it closer toward you, or yourself closer to it, but doing so doesn’t render anyone or anything any lighter or bestow any other properties onto anything.
Lighten
While the master moonstone is affixed to an item, that item’s weight is reduced by 1,000 lbs. If you affix the master moonstone to a container, whether on the outside or on the inside, the weight of the entire container is reduced by 1,000 lbs. (However, if the container is a magical one that ignores the weight of its contents, such as a bag of holding, nothing happens.)
It is possible to reduce an item’s effective weight to 0 (but never to less than 0).
When the object is a vehicle, its speed is increased by 2 and its carrying capacity is increased by 1,000 lbs.
Levitate
If a master moonstone is not affixed to anything and nothing or nobody is holding the chain in place, the moonstone floats off into the sky, or against the ceiling if indoors, dealing it 5 points of damage, or 10 if the ceiling is 10 feet or more above it. It continues to ascend at a speed of 50 feet (10 squares) per round until it encounters the ceiling, someone grabs the chain, or it is 500 ft above the ground, at which point it stops moving. When the master moonstone is up in the air and has stopped moving, you can climb the chain (Climb DC 10) or force it back down 5 feet (Str DC 21)
Master Moonstone Empowerment
If two master moonstones are affixed to an object, and the object combined with all the creatures and cargo currently on and/or inside it weighs no more than 3,000 lbs, that object starts to hover exactly a foot above the ground (or, when over water, above the surface). If the object combined with all the creatures and cargo weighs no more than 2,000 lbs, that object starts floating like a master moonstone does.
When the object is a vehicle that can still move while not touching the ground (or the water), its hover or fly speed is the same as that of its regular movement. While flying or hovering, the vehicle ignores difficult terrain (but creatures pushing or pulling the vehicle don’t).

***

As you can see, the possibility of accidentally making yourself fly away has been removed, and with it, sadly, the ability to do the same to your opponent – at least, unless the opponent weighs less than 40 pounds, like a pixie, an imp, or an ancient unkillable demon in the shape of a scrawny seven-year-old boy.

I think, for now, I’ll give the PCs something like one master moonstone, two major moonstones, and somewhere around ten minor moonstones, and somewhere down the line they might run into another master moonstone, already installed on a ship, and at least one other major moonstone.
Until then, I’ll just see how things play out. When they get to the treasure room, they have to break out again, so that’s where the reverse pile driver thing might come in. After that, Maybe the PCs will decide that they like having the moonstone more than they would like having its value in gold, but they don’t have to. If they start trying to sell it, it’ll still take a while to find a buyer, and in the meantime, I think the moonstone is a more interesting treasure than a mountain of generic valuables, and a more convenient one to carry around - for that reason alone I think it’s worthwhile to introduce it to the campaign, to answer Adoendithas’ question.

I’ll have to get back to the issue of how much the moonstones are worth later.

Questions, advice, feedback, corrections et cetera continue to be appreciated and borrowing is cordially invited, especially if you let me know how it goes.:smallbiggrin:

AcerbicOrb
2013-02-02, 10:08 AM
This seems really interesting and well thought out. I can just picture a huge castle floating into the atmosphere after taking a battering from heavy siege weapons, or a hero jumping onto a chandelier to hide and set up an ambush. It has loads of possibilities.

Harkoth
2013-02-06, 11:43 AM
The old guy in UP did this already and only used a few hundred helium balloons. And you only need something like a garden hose and a fat kid to make an anchor, I think.

Adoendithas
2013-02-06, 05:29 PM
The old guy in UP did this already and only used a few hundred helium balloons. And you only need something like a garden hose and a fat kid to make an anchor, I think.

Helium apparently can lift about one gram per liter, so a 50kg person would need about 4,000 helium balloons to take off. A house would need way more. And the movie didn't explain how he managed to keep his house on the ground until he was ready to take off. :smalltongue:

Shining Wrath
2013-02-07, 08:49 PM
You have to decide why your moonstones try to move away from the surface. If they have negative mass, they are repelled by anything with positive mass but would be drawn toward each other. As an interesting side effect, if you threw one at someone and scored a hit, they would jerk the person toward you as though you had pulled on them at the point of impact.

If they are buoyant than they will lose buoyancy as they ascend in the atmosphere, and there will be a (somewhat variable) altitude where they level off. This appears to be the choice you have made.

Your moonstones are very nearly equivalent to helium balloons but they have more lift per unit volume.

I congratulate you on a fun idea.

Adoendithas
2013-02-08, 08:44 AM
If they have negative mass...

Of course if you have negative mass you're already wrecking all of Newton's gravity stuff, so there's no way to tell exactly WHAT they would do when you applied a force to them.

hamishspence
2013-02-09, 05:41 AM
Actually, all wormhole and FTL (faster than light travel) theories tend to begin with the assumption that you've got exotic matter to work with. Which has- negative mass.

So- could do some interesting things with these moonstones. Use them in the manufacture of portals, say.

Adoendithas
2013-02-09, 10:12 AM
Actually, all wormhole and FTL (faster than light travel) theories tend to begin with the assumption that you've got exotic matter to work with. Which has- negative mass.

So- could do some interesting things with these moonstones. Use them in the manufacture of portals, say.

I'd never thought of that--they'd also cancel out some of the weird things that could happen to gravity around a portal.

Synovia
2013-02-09, 11:37 AM
You have to decide why your moonstones try to move away from the surface. If they have negative mass, they are repelled by anything with positive mass but would be drawn toward each other. As an interesting side effect, if you threw one at someone and scored a hit, they would jerk the person toward you as though you had pulled on them at the point of impact..

They don't have negative mass. They have negative weight. There's a very important distinction there.

Something with negative weight is something with density less than air. It'll float on air like a beachball does on water.

hamishspence
2013-02-09, 03:13 PM
Even that has weight- in a zero-atmosphere environment, it will sit on the scales, and its weight can be measured.

Analytica
2013-02-09, 03:42 PM
Maybe their negative weight status is relative to where they are in relation to the ground? I.e. they would have zero weight wrt. the earth at some point in orbit, and positive weight wrt. the earth outside orbit, but negative weight wrt. the earth below this height. You would then get moonstones falling up or down to form a band around the earth, wherein they would float. Possibly different isotopes or the like have different stable orbit heights, so you can calibrate blends of them to make floating things at different heights. Of course, this gets much closer to Aristotelian than Newtonian physics, but...

Adoendithas
2013-02-09, 06:03 PM
Maybe their negative weight status is relative to where they are in relation to the ground? I.e. they would have zero weight wrt. the earth at some point in orbit, and positive weight wrt. the earth outside orbit, but negative weight wrt. the earth below this height. You would then get moonstones falling up or down to form a band around the earth, wherein they would float. Possibly different isotopes or the like have different stable orbit heights, so you can calibrate blends of them to make floating things at different heights. Of course, this gets much closer to Aristotelian than Newtonian physics, but...

Of course, D&D uses the four classical elements, so why not?

hamishspence
2013-02-09, 06:14 PM
I rather liked Mimir.net's hybrid of atomic theory and classical theory, for Planescape:

http://mimir.net/essays/planarphysics.html

Adoendithas
2013-02-10, 09:24 AM
That is awesome, WotC needs to add it to the Manual of the Planes.

Shining Wrath
2013-02-11, 05:00 PM
Of course if you have negative mass you're already wrecking all of Newton's gravity stuff, so there's no way to tell exactly WHAT they would do when you applied a force to them.

Work with me here. F=ma, and m has a negative value.

Shining Wrath
2013-02-11, 05:03 PM
They don't have negative mass. They have negative weight. There's a very important distinction there.

Something with negative weight is something with density less than air. It'll float on air like a beachball does on water.

That was my point, actually. Negative mass is different than buoyancy, and the OP went with buoyancy.

Shining Wrath
2013-02-11, 05:06 PM
Maybe their negative weight status is relative to where they are in relation to the ground? I.e. they would have zero weight wrt. the earth at some point in orbit, and positive weight wrt. the earth outside orbit, but negative weight wrt. the earth below this height. You would then get moonstones falling up or down to form a band around the earth, wherein they would float. Possibly different isotopes or the like have different stable orbit heights, so you can calibrate blends of them to make floating things at different heights. Of course, this gets much closer to Aristotelian than Newtonian physics, but...

The real world formula for gravitational attraction is F=Gm1m2/r^2, where
F=force of attraction, aka weight
G=a constant which makes the units come out "right"
m1=mass of the planet
m2=mass of the object
r=the distance between the center of mass of the planet, and the center of mass of the object

Clearly as r increases the attraction to the object decreases. Now, if we go back to moonstones having negative mass, then the sign of "F" would be negative, and would become less negative as you moved away from the planet.

Adoendithas
2013-02-11, 05:28 PM
Work with me here. F=ma, and m has a negative value.

But that would make some things become really strange, since the force and the acceleration are now opposites. If you put a regular object on a table (which exerts a normal force) that force should counteract gravity, but with a moonstone (on the bottom of the table) it would be literally sucked into the wood. If you pushed one with your hand it would move in the opposite direction, trying to go through your palm, and friction would make it speed up rather than slow down--it would actually speed up more and more as it went, since the friction increases as velocity does.

Tegu8788
2013-02-11, 10:08 PM
He didn't come up with the situation, he just applied physics to it.

Synovia
2013-02-13, 04:29 PM
Even that has weight- in a zero-atmosphere environment, it will sit on the scales, and its weight can be measured.

Weight is the force acting on an object produced by gravity. In a zero-G environment, something has no weight. Weight is intrinsically relative.

What you're thinking about is MASS. Particles (except in weird exceptions) always have mass.

Shining Wrath
2013-02-14, 11:11 AM
But that would make some things become really strange, since the force and the acceleration are now opposites. If you put a regular object on a table (which exerts a normal force) that force should counteract gravity, but with a moonstone (on the bottom of the table) it would be literally sucked into the wood. If you pushed one with your hand it would move in the opposite direction, trying to go through your palm, and friction would make it speed up rather than slow down--it would actually speed up more and more as it went, since the friction increases as velocity does.

Negative mass makes life quite interesting. Maybe that's why it's only found in fantasy :smallsmile:

hamishspence
2013-02-14, 02:08 PM
Weight is the force acting on an object produced by gravity. In a zero-G environment, something has no weight. Weight is intrinsically relative.

I said Zero-atmosphere, not zero-G. Like the Moon, or an asteroid.

"Weight is what you measure when you put something on a set of scales" so to speak. Mass is, as you say- always the same.