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Fenix_of_Doom
2007-09-22, 03:21 PM
Some people start complaining when you mix physics with magic, if your one of those people, get out of this thread.

At Fax's maga-crab (http://www.giantitp.com/forums/showthread.php?t=57301) thread my attention was drawn to an interesting physics debate, I however do not wish to derail that thread so I made a new one, I'll give you the start-up.

A small pile of pebbles does not cause 1000 damage. 'Tossing them onto a solid surface' doesn't necessarily include the tarrasque itself (leaving aside the assumption that the boulders retain the same velocity as the pebbles).

After that, it's pure physics, unless you think that heavy stone won't fall, for somewhat reason. Why, they suddenly turn anti-grav or something? They will actually fall faster, not slower.
Unless you have gm that magically turns them into the styrofoam or haven't learned physics at all, that results will be exactly the same under any sane gm - several tons on heavy matter splatting one crab.
emphasis his, but very fitting in context.

Psst- all things fall at the same speed regardless of weight. Simple phyics :P

Pssst - that works only in open space. Simple physics :smallamused:

And now, go and make your homework on effects of air friction and how volume relates to surface area :smallyuk:

and why dont you go home and calculate how little effect air friction actualy has on something as dense as stone?

You know, your little theory [Aristotelian, btw] was refuted by guy named Galileo Galilei almost 500 years ago.
Yeah, go ahed, pick one stone weighting 100 g and other weighting 1 kg and drop them from a reasonable height.
You know, level of teaching has fallen significantly during the last 20 years, what I was in elementary school - that was one of the first physics experiments conducted here, and to not kno... ech, whatever. :smallannoyed:
If you guys are reading what I'm reading then Trixie has now proven himself wrong, congrats on that BTW.

But that was just for fun, I'm actually going to ignore all that and instead I'll try to guess what Zincorium meant.
When magic and physics meet things sometimes get a little difficult, so let's start at the basics, when an object falls it will gain speed thus it gains kinetic energy. The formula for kinetic energy is as following: E=0.5*m*v^2
this means that if the mass of the falling pebbles would increase by a factor of 4.000 (http://www.d20srd.org/srd/spells/shrinkItem.htm) and the energy constant that the speed would decrease significantly.
in context of the pebble throwing this would be no problem if the command word was use at the same time you drop the pebbles but it would be a problem if you use the return to original size when it hits something method.

Now I know myself, so chances are I made a mistake or missed a factor somewhere, so comment or post your own magic/physics calculations here.

goat
2007-09-22, 03:35 PM
Well, the magic effectively produces mass from "nowhere".

If the "nowhere" views the shrunken object as being stationary, and simply introduces the mass into the same reference frame as the object, completely "ignoring" the rest of the universe around it, then it would make sense for it to be initially travelling at the same speed as the small object from the viewpoint of an external observer. It would then accelerate at whatever rate was appropriate for the external conditions.

daggaz
2007-09-22, 03:57 PM
Well Fenix, your math is correct. You are going off of the principal of Conservation of Energy, from which the Conservation of Momentum (mass*velocity) is derived.

Of course, suddenly pounding a ton of extra mass in magically violates the conservation of energy by default, but like you said, its magic.

So yeah, if you allow the magical introduction of mass, but want to conserve energy after the fact, then the resulting velocity will be reduced. Also, there is a huge problem with the fact that the mass is instantaneously introduced (leading to an instantaneous shift in velocity and therefore acceleration) but that is a singularity that can be just as well ignored and described away by magic. Note that the velocity cannot be reduced to exactly zero, regardless of the mass increase. Also note that gravity will begin accelerating the new mass immediately, so its not like the boulder is gonna just hang in mid air.

And for the record, that guy did prove himself wrong. Quite funny, actually.
Gravity accelerates all mass at the same rate, depending on the mass of the two interacting bodies, based off the gravitational constant 'G'. On earth, this results in a more or less constant value of 'g', or 9.81.. etc.. meters per second squared. Its not an exact constant tho, and ultrasonic aircraft actually make use of gravitational maps when flying their highspeed missions, especially over large mountain ranges and oceans, where the differences are most prominent.

goat
2007-09-22, 04:02 PM
Also note that gravity will begin accelerating the new mass immediately, so its not like the boulder is gonna just hang in mid air.

Actually, in theory it should. Gravity is still bound by a maximum speed of c.

Edit - I mean, the distance between the new mass and some (ANY) existing mass will likely make that a hideously small amount of time. But for a tiny fraction of a second, it should just sit there.

Dervag
2007-09-22, 04:16 PM
Actually, in theory it should. Gravity is still bound by a maximum speed of c.

Edit - I mean, the distance between the new mass and some (ANY) existing mass will likely make that a hideously small amount of time. But for a tiny fraction of a second, it should just sit there.You mean the Looney Toons artists were right all along?

Yaay!

Zincorium
2007-09-22, 04:23 PM
Not much need to guess, really.

I was going off of the 'pebbles hitting a solid surface' trigger, where the pebbles remain pebbles until they have hit ground and stopped. Going by this version of the spell, there is no acceleration* of the boulders once they've changed from pebbles.

If they've hit the ground, the energy of the pebbles is so low that when applied to the much greater mass of the boulders, the impact would be less than impressive.

Edit:
*Acceleration in the layman sense. I know the reduction in speed caused by the pebbles hitting ground is also a type of acceleration. But the people who know that probably understand what I was trying to say anyway.

daggaz
2007-09-22, 04:30 PM
Actually, in theory it should. Gravity is still bound by a maximum speed of c.

Edit - I mean, the distance between the new mass and some (ANY) existing mass will likely make that a hideously small amount of time. But for a tiny fraction of a second, it should just sit there.

We are talking on the order of picoseconds, or probably less. This is less than the time it takes interatomic interactions to occur. Symantics aside, the rock will begin to fall immediately. In fact, the rock will begin to fall in less time than it takes light to bounced back and forth, and in far less time than your optical cells can fire nerve signals off in, let alone before they reach your brain.

Kaelik
2007-09-22, 04:32 PM
Well the big question is terminal velocity. It doesn't matter how fast gravity accelerates an object if it has time to reach terminal velocity. I would think that with the height we are talking about it would certainly reach that level. My big question is does Shrink Item decrease it's mass?

Assuming it does then the terminal velocity of (lets pretend) a sphere is going to be decreased by the larger diameter, but increased by the larger mass. I therefore don't know how to compare the terminal velocities of the two particles.

goat
2007-09-22, 04:39 PM
I was going off of the 'pebbles hitting a solid surface' trigger, where the pebbles remain pebbles until they have hit ground and stopped. Going by this version of the spell, there is no acceleration* of the boulders once they've changed from pebbles.

Ah! So, in this trigger, does the magic occur at the instant of the first point of contact, or once the energy has been transferred?

If the pebble expands instantaneously (which is only as absurd as the mass from nowhere) upon any point of contact, the rear of the pebble may not yet have stopped moving, and still contain all of its initial kinetic energy! If the expanded rock is moving at the same speed, it will still have a massive amount of force to bring to a rest.

BUT, this brings other problems to mind. Which way does the pebble expand? Does it "explode" out in all directions? Grow only away from the struck surface? I'm not actually sure the spell description provides information on what happens if it tries to expand into too small a gap.

Fenix_of_Doom
2007-09-22, 04:39 PM
Not much need to guess, really.

It was guessing because I'm not a mind reader, but it was what I like to call a calculated guess.
As the rules don't mention anything like this you could technically use any of the following rulings:
1 the new mass has the same relative energy as it had when you cast the spell
2 the new mass has the same relative energy as the shrinked remains
3 the new mass has no energy at all since the spell doesn't mention it does(so cold damage then?:smalltongue: ).

edit:

My big question is does Shrink Item decrease it's mass?

yes it does, by a factor 4000 no less, check my original post for the link.

goat
2007-09-22, 04:41 PM
Symantics aside...

Good lord man! This is theoretical physics! Semantics can NEVER be put aside.

Zincorium
2007-09-22, 04:48 PM
It was guessing because I'm not a mind reader, but it was what I like to call a calculated guess.

I just meant you could, y'know, ask me what I meant. Given that what someone means is very hard to calculate.

My belief, and my point, is that the speed of the pebble will probably not be retained, only the kinetic energy possessed by that pebble, which when the greater mass is taken into effect, results in a very slow downward moving boulder.

Really, the fact that you have a boulder lying on top of you would outweigh (no pun intended) any kinetic energy that the boulder retained from the pebble.

This is assuming that by hitting they mean any contact with a solid surface. If they meant that it activates when the object is resting on a solid surface, and they very well might have, then it is of course going to be motionless.

daggaz
2007-09-22, 04:51 PM
Assuming it does then the terminal velocity of (lets pretend) a sphere is going to be decreased by the larger diameter, but increased by the larger mass. I therefore don't know how to compare the terminal velocities of the two particles.

Not true, sorry. Terminal velocity is a function only of the surface area and subsequent drag of a material. Gravity accelerates two different masses at exactly the same rate, regardless of size or shape. But the air does play a part (note that now we arent talking about the accelerating force, but the decelerating force, or drag).

Aerodynamic drag has the most to do with the shape and, especially, the surface area of the falling object. It is also directly related to the square of the velocity. The ultimate velocity is independant of mass. So, two different massed objects will speed up to a certain point, but at some point the velocity^2 * surface area begins to quickly overtake the force of gravity, and you end up with an equilibrium falling speed.

Obviously, items like a human body or a parachute or much harder to model mathematically, but if you take the case of two spheres its much easier. Two spheres of the same size and different mass will fall at exactly the same rate, and reach the same exact terminal velocity. Two spheres of different sizes and the same mass will reach different terminal velocities.

This is why the terminal velocity for human beings (i think its about 144 miles per hour, but i might be mistaken) is only an estimate. You are assuming roughly the same shape and size, and disregarding the differences in mass. (think of your friends at school who are more or less the same size as you, but might be a bit more heavierset).

Ulzgoroth
2007-09-22, 04:54 PM
It might reasonably conserve momentum instead. Since we cheerfully disconserve both with various spells (at least from a naive view of local physics) expecting either one to prevail seems equally valid. Conservation of momentum would make the KE actually go down when the rock un-shrinks.

It could conserve velocity instead, but that would make abuse trivial. Just toss and talk...

Of course, the intended trick was to un-shrink the stone and let it fall, which works fine with any of those.

bosssmiley
2007-09-22, 04:55 PM
Fenix - did you honestly hive off a whole new thread just to up the count on your catgirl cull? Are you trying to powerlevel by rampant nekocide or something? :smallwink:

2007-09-22, 04:56 PM
"When magic and physics meet things sometimes get a little difficult, so let's start at the basics, when an object falls it will gain speed thus it gains kinetic energy. The formula for kinetic energy is as following: E=0.5*m*v^2
this means that if the mass of the falling pebbles would increase by a factor of 4.000 (http://www.d20srd.org/srd/spells/shrinkItem.htm) and the energy constant that the speed would decrease significantly."(this is supposed to be a quote)

That the stone would decrease in velocity would be a violation of newtons first law( an object will reman at a constant speed or at rest unless an external force acts upon it.) As no forces are added by the spell, it would continue to accellerate downwards until reaching terminal velocity.

Ulzgoroth
2007-09-22, 04:57 PM
Not true, sorry. Terminal velocity is a function only of the surface area and subsequent drag of a material. Gravity accelerates two different masses at exactly the same rate, regardless of size or shape. But the air does play a part (note that now we arent talking about the accelerating force, but the decelerating force, or drag).

Aerodynamic drag has the most to do with the shape and, especially, the surface area of the falling object. It is also directly related to the square of the velocity. The ultimate velocity is independant of mass. So, two different massed objects will speed up to a certain point, but at some point the velocity^2 * surface area begins to quickly overtake the force of gravity, and you end up with an equilibrium falling speed.
But the force of gravity (not the acceleration from gravity) is proportional to mass. Two outwardly identical objects will experience the same drag when falling at the same speed, but if one has more mass it will experience more gravitational force and thus have a higher terminal velocity.

goat
2007-09-22, 05:00 PM
But the force of gravity (not the acceleration from gravity) is proportional to mass. Two outwardly identical objects will experience the same drag when falling at the same speed, but if one has more mass it will experience more gravitational force and thus have a higher terminal velocity.

This is very true, and just like my point above about the speed of gravity, completely meaningless in this conversation.

Excellently done.

edit - I say completely meaningless, I suppose if we were dealing with a block of degenerate neutron matter it could be important. Two dozen cubic feet of that would weigh over 50*10^15 kilos, enough for significance.

edit 2 - I say significant, thinking about it, it would only be another few micrometers a second...

Zincorium
2007-09-22, 05:16 PM
That the stone would decrease in velocity would be a violation of newtons first law( an object will reman at a constant speed or at rest unless an external force acts upon it.) As no forces are added by the spell, it would continue to accellerate downwards until reaching terminal velocity.

Objects in motion will remain in motion until acted on by a net force. Speed and what constitutes an outside force are not part of the first law.

Momentum is conserved, and as momentum is a product of mass and velocity, a change in mass would violate the conservation of momentum unless the velocity drops. The object in question does remain in motion, albeit drastically slowed from an outside perspective, so no violation of newton's first law occurs.

daggaz
2007-09-22, 05:24 PM
But the force of gravity (not the acceleration from gravity) is proportional to mass. Two outwardly identical objects will experience the same drag when falling at the same speed, but if one has more mass it will experience more gravitational force and thus have a higher terminal velocity.

The problem is one of limiting factors. The point is, the velocity squared term completely outstrips differences in mass, so a small (or even fairly large) difference in mass between two objects has a very negligable effect on terminal velocity. Remember, when dealing with two diff masses, say 50 lbs and 200 lbs, its not the difference between them, its the difference between the mass of the entire earth and the object, and the same difference from the other object. Because of how you compute g, this comes out very very close, whereas the aerodynamic drag factor completely outstrips this term with only small changes in surface area. (remember,
g is G*(m_object*mass_earth)/r^2 (and G is really really small) while the drag is a coefficient*v^2.) That is why you have a general terminal velocity for anything, regardless of mass, as long as it shares a close enough shape and surface area. Parachutes and feathers and falling pieces of paper are obvious deviations due to the dramatically different shape and surface area, as well as the extremely light mass.

Sorry Goat, but you are wrong as far as practical interpretations go. (although entirely correct, if you count things out to the further decimal places... problem with this is, the uncertainty in the measurements often exceeds this very small margin of proof. In other words, a heavier mass will have a faster terminal velocity technically), you just cant measure it accurately.

Chronos
2007-09-22, 07:00 PM
The problem is one of limiting factors. The point is, the velocity squared term completely outstrips differences in mass, so a small (or even fairly large) difference in mass between two objects has a very negligable effect on terminal velocity.Uh, no. Terminal velocity has nothing to do with energy, only with forces. The drag force on an object is equal to the object's surface area times its velocity squared times a factor which depends on the object's shape. The force of gravity is equal to the mass of the object times the acceleration due to gravity. Terminal velocity is the velocity at which these two forces are equal: s*A*v^2 = m*g. Solving this for v, we have v = sqrt((m*g)/(s*A)). In other words, if you have two objects with the same shape and area, but different masses, then the heavier one will have a higher terminal velocity, in proportion to the square root of the mass ratio. Thus, for instance, if the heavier object has four times the mass of the lighter one, it'll fall twice as fast. If the heavier object has 100 times the mass, it'll fall 10 times as fast, and so on.

In the case we're looking at here, each of the object's linear dimensions increases by a factor of 16, so its area increases by a factor of 256. Its mass, meanwhile, is proportional to its volume, and thus increases by a factor of 4096 (this is the "approximately 4000" referred to by the spell). So the boulder will have a terminal speed of sqrt(4096/256) times that of the pebble, or 4 times.

Ulzgoroth
2007-09-22, 07:04 PM
I'm not going to dispute your aerodynamics, but the math you're doing with them is wrong. Likewise, your conclusions from the formula for gravitational force (not g...g is gravitational acceleration normally).

The formula is correct, but if you actually look at what it says, that 200 lb object has exactly 4 times as much gravitational force on it as the 50 lb object. And thus the same acceleration, of course. If a factor of 4 is trivial is trivial to you...:smallconfused: The difference of mass between the object and the earth is utterly irrelevant, unless you think I'm factoring in the acceleration of the earth toward the falling object (I'm not). Which exists, but is trivial for reasonable masses.

If drag is indeed proportional to v^2, and these two masses have the same coefficient, the heavier one will have twice the terminal velocity. This is trivial. There is no such thing as a 'general terminal velocity'. For objects of identical surface, terminal velocity rises as the square root of mass.

Kurald Galain
2007-09-23, 02:11 AM
Aside from all the catgirl splattage, Mage: the Ascension has the best answer to this. Physics is just another kind of magic, that happens to work because the Order of Reason convinced most of humanity that Reality does indeed work like that. Which means that in certain deserted areas, your jet plane will be taken down by a flying carpet, because Consensual Reality over there has a different paradigma.

Leon
2007-09-23, 02:52 AM
a large supply of female humanoid cats

2007-09-23, 06:13 AM
Objects in motion will remain in motion until acted on by a net force. Speed and what constitutes an outside force are not part of the first law.

Momentum is conserved, and as momentum is a product of mass and velocity, a change in mass would violate the conservation of momentum unless the velocity drops. The object in question does remain in motion, albeit drastically slowed from an outside perspective, so no violation of newton's first law occurs.

I disagree, for an object to change in velocity it must either accellerate or decellerate(duh). Accelleration = Force/mass. hence if force = 0, accelleration(or decelleration in this case) equals zero, and hence the speed of the object would not change.

Besides Newton in his first law emphesises constant velocity, hence if the speed has changed a force needs to be applied.

daggaz
2007-09-23, 06:35 AM
I disagree, for an object to change in velocity it must either accellerate or decellerate(duh). Accelleration = Force/mass. hence if force = 0, accelleration(or decelleration in this case) equals zero, and hence the speed of the object would not change.

Besides Newton in his first law emphesises constant velocity, hence if the speed has changed a force needs to be applied.

Zincorium is entirely correct if you accept in this world of magic
that conservation of energy, and thus, momentum, is conserved. If the energy is conserved, the object WILL slow down when it's mass is increased. That is pure and simple physics. But if we dont follow the conservation of energy, anything can happen, it is up to us to define things.

And that is the crux of this entire thread. Without defining the paramaters where physics and magic meet, we really have nothing to discuss without endless arguments, as everybody will be arguing about different things. (unless they are arguing about pure physics, as those parameters have already been defined).

EDIT oh and ha ha, sorry for the above posts. That is what you get when you argue physics while incredibly drunk (sorry, we had a huge dorm party last night)

2007-09-23, 06:59 AM
Zincorium is entirely correct if you accept in this world of magic
that conservation of energy, and thus, momentum, is conserved. If the energy is conserved, the object WILL slow down when it's mass is increased. That is pure and simple physics. But if we dont follow the conservation of energy, anything can happen, it is up to us to define things.

last night)

I disagree that it is that simple. The earth velocity towards the rock could increase instead, since that would still mean total momentum was conserved.

Even though this would have the same problem as previously with velocity changing without a force, which would be evidence for your statement that without parameters we cannot get any clear answer, as either the law of conversion of momentum is broken, are newtons first and second laws would be broken.

Fenix_of_Doom
2007-09-23, 08:07 AM
Fenix - did you honestly hive off a whole new thread just to up the count on your catgirl cull? Are you trying to powerlevel by rampant nekocide or something? :smallwink:

Yes, that's the true reason didn't you hear the catgirls are taking over the world with the new 4th edition, we MUST STOP THEM:smallwink: .

I disagree, for an object to change in velocity it must either accellerate or decellerate(duh). Accelleration = Force/mass. hence if force = 0, accelleration(or decelleration in this case) equals zero, and hence the speed of the object would not change.

Besides Newton in his first law emphesises constant velocity, hence if the speed has changed a force needs to be applied.

it only keeps constant velocity if unaffected by outside forces, I would say the release of a shrink spell is definitely an outside force.

Citizen Joe
2007-09-23, 09:43 AM
You can draw energy from a source other than kinetic.

When you let compressed air expand rapidly, the temperature drops. Thus the pebble to boulder may result in the boulder suddenly becoming much colder.

You may draw from the nuclear forces that hold the matter together. This actually works best to negate the cheese. I.e. To expand the boulder's mass, the strong forces within the pebble become very weakened. So much that on impact, it shatters easily into powder. Thus doing no more damage than a mere pebble would have done.

Enzario
2007-09-23, 10:32 AM
Besides Newton in his first law emphesises constant velocity, hence if the speed has changed a force needs to be applied.

Aye, but Newton, though ahead of his time, did not catch up to 21st century advances in physics. One question you must look at in this situation is what will remain constant: kinetic energy or velocity? In some ways, we must assume that kinetic energy stays the same, since the spell only adds MASS, not energy. In that case then, you would have the same amount of energy propelling the pebbles (your toss) being transferred into boulders. The boulders would then hit with the same force of your (underhand?) toss, plus the energy gained by acceleration due to gravity in the time since their expansion; if you set the trigger for their expansion to be their impact on said tarrasque, balor, PPDC, etc., the only damage you would be doing would be the placement of said large rocks on top of them.

All done in the name of research. If the catgirls ever take over the world, I'll be tried for my war crimes. (jar of cookies if you can get that book reference; hint: Michael Crichton)

Arbitrarity
2007-09-23, 10:49 AM
So as to not conflict with potential rulings on conservation of momentum, my "Rocks fall" method involves shrinking your items, creating a Reverse Gravity field, dropping my items in it, and Disjunctioning the entire thing, so very large rocks fall.

Dervag
2007-09-23, 11:17 AM
Fenix - did you honestly hive off a whole new thread just to up the count on your catgirl cull? Are you trying to powerlevel by rampant nekocide or something? :smallwink:Personally, I think that catgirls are continuously being created and destroyed, and that this is an important part of the cycle of life.

That the stone would decrease in velocity would be a violation of newtons first law( an object will reman at a constant speed or at rest unless an external force acts upon it.) As no forces are added by the spell, it would continue to accellerate downwards until reaching terminal velocity.It is not the case that no force acts on the object, or at least it doesn't have to be.

For example, imagine a falling rock which is made out of atoms. Now imagine that I start 'adding' atoms to the rock that are stationary relative to the surroundings, and not to the rock. There are several ways for me to do this. I can spray the atoms on from underneath using a spray can. I can somehow warp them in right into the internal structure of the rock. But however I do it, the fact remains that atoms not moving relative to the ground and atoms moving relative to the ground are now in close proximity, bonded together.

So there is a force- the force required to accelerate all those atoms up to the speed of the rock in a big hurry. Otherwise, you can't add any mass to the rock.

So if we assume that the extra mass introduced does not come into being with the same velocity as the old mass, and is stationary in the frame of reference of the surroundings rather than of the rock, it is entirely reasonable for adding more weight to slow down the rock.

But the force of gravity (not the acceleration from gravity) is proportional to mass. Two outwardly identical objects will experience the same drag when falling at the same speed, but if one has more mass it will experience more gravitational force and thus have a higher terminal velocity.

This is very true, and just like my point above about the speed of gravity, completely meaningless in this conversation.

Excellently done.

edit - I say completely meaningless, I suppose if we were dealing with a block of degenerate neutron matter it could be important. Two dozen cubic feet of that would weigh over 50*10^15 kilos, enough for significance.

edit 2 - I say significant, thinking about it, it would only be another few micrometers a second...No, he's right. Denser objects are relatively less affected by air resistance. A sphere of iron and a hollow sphere of paper will not fall at the same speed in an atmosphere; the paper sphere will reach terminal velocity much sooner while the sphere of iron will continue to accelerate towards the ground for longer and therefore hit much harder.

Momentum is conserved, and as momentum is a product of mass and velocity, a change in mass would violate the conservation of momentum unless the velocity drops. The object in question does remain in motion, albeit drastically slowed from an outside perspective, so no violation of newton's first law occurs.Moreover, Newton's First Law isn't sacred. It's really just a special case of Newton's Second Law, but one that is very important as a concept because ignoring it makes a hash of the theory.

When 'real' physicists write Newton's Laws in mathematical form, they only use two equations- one for Newton's Second and one for Newton's Third, because the First follows logically from the Second as long as you use a solid definition of what things like 'velocity' and 'force' mean. Newton couldn't guarantee that his readers would use solid definitions, so he had to make sure that the idea of things not accelerating when nothing was pushing them was nailed down firmly.

You may draw from the nuclear forces that hold the matter together. This actually works best to negate the cheese. I.e. To expand the boulder's mass, the strong forces within the pebble become very weakened. So much that on impact, it shatters easily into powder. Thus doing no more damage than a mere pebble would have done.If you weaken the strong nuclear force much, the pebble undergoes nuclear fission and releases way more energy than you want. Fission happens when a nucleus gets too big for the strong force to hold it together.

Better to weaken the weak nuclear force, the one that holds the atoms together. At low (i.e. nonrelativistic) speeds, either force has enough energy to provide an extra wallop of kinetic energy.

Personally, I'd just stick with conservation of momentum. I can explain apparent nonconservation of momentum through field effects*, but I can't explain nonconservation of energy... well, actually a working definition of 'magic' is 'anything that creates apparent violations of the law of conservation of energy or the Second Law of Thermodynamics'.

So I guess it's a wash.

The boulders would then hit with the same force of your (underhand?) toss, plus the energy gained by acceleration due to gravity in the time since their expansion; if you set the trigger for their expansion to be their impact on said tarrasque, balor, PPDC, etc., the only damage you would be doing would be the placement of said large rocks on top of them.Your reasoning is very good, but you shouldn't use 'force' in the first sentence I'm quoting. Force is a technical term, remember?

When talking in abstract terms about the damage caused by a collision, I try to avoid using any technical term to cover the abstract damage. I use words like 'oomph', 'wallop', and 'smackage.' That way I can use words like force if and only if they actually fit the definition (i.e. mass times acceleration).

So as to not conflict with potential rulings on conservation of momentum, my "Rocks fall" method involves shrinking your items, creating a Reverse Gravity field, dropping my items in it, and Disjunctioning the entire thing, so very large rocks fall.That's cool. If you can turn off the gravity during the process of increasing mass, then they'll hit just as similarly large boulders would dropped from whatever height they were enlarged at. Of course, that's what they'd do anyway...

However, you still have a momentum conservation problem. In this case, you have an angular momentum conservation problem, assuming that you're doing all this on a body that is orbiting something.

Which you might not be; I can and have imagined a D&D campaign taking place on a thousand-mile thick pancake of rock occupying a piece of the inside of a Dyson sphere.

Citizen Joe
2007-09-23, 11:39 AM
We could also apply Star Trek physics to the pebble/boulder... when expanded, they actually turn into styrofoam and bounce off harmlessly.

I think the strong anti-cheese forces within a boulder could be used to prevent them from being abused with shrink item. Since compression causes an increase in temperature, shrinking a suitably sized boulder may cause it to immediately burst into flames/shatter. Likewise the expansion could cause it to suddenly chill. The strong anti-cheese force exists to prevent the interaction of magic and physics, thus ruining most abuses of the system.

Tor the Fallen
2007-09-23, 12:31 PM
So could you use enlarge person as slow fall?

Fenix_of_Doom
2007-09-23, 01:18 PM
So could you use enlarge person as slow fall?

It would, but enlarge person only increases mass by a factor 8 and the only reduces the fall speed with a factor of sqrt8(roughly 2.8), so you should still take some damage.

I can't wait to try this out.

Chronos
2007-09-23, 01:47 PM
Enlarge Person would increase falling damage; Reduce Person would decrease it. Or at least, it would, if the D&D rules paid heed to physics and changed falling damage according to size, but they don't.

Trying to get D&D magic consistent with the laws of physics, particularly the conservation laws, is hopeless. For any conservation law you name, I can find a spell or magic item which violates it (well, maybe not any conservation law: I don't think there are any spells which violate Conservation of Strangeness, say, but certainly Mass, Energy, Momentum, and Angular Momentum are out the window). So when we have a spell which clearly violates at least one of the laws, I don't think it's meaningful to try to figure out which one it violates: For all we can tell from physics alone, it might violate all of them. The only hope is if the D&D rules address a situation.

And, as it happens, the rules do address the possibility of a projectile changing size in flight. From the text of spells like Enlarge Person (http://www.d20srd.org/srd/spells/enlargePerson.htm) and Righteous Might (http://www.d20srd.org/srd/spells/righteousMight.htm), we learn that thrown objects do damage according to their size on impact, but that projectiles (launched from a bow, sling, or similar) do damage according to their size at launch. This does leave one wondering how the laws which govern the universe of a D&D world distinguish between an arm and any other launch mechanism, but regardless of how it works, the rules tell us that it does. So for Shrink Item, you just have to make sure that you throw your weaponized pebbles by hand, not with a projectile weapon.

Tor the Fallen
2007-09-23, 01:50 PM
Enlarge Person would increase falling damage; Reduce Person would decrease it. Or at least, it would, if the D&D rules paid heed to physics and changed falling damage according to size, but they don't.

Trying to get D&D magic consistent with the laws of physics, particularly the conservation laws, is hopeless. For any conservation law you name, I can find a spell or magic item which violates it (well, maybe not any conservation law: I don't think there are any spells which violate Conservation of Strangeness, say, but certainly Mass, Energy, Momentum, and Angular Momentum are out the window). So when we have a spell which clearly violates at least one of the laws, I don't think it's meaningful to try to figure out which one it violates: For all we can tell from physics alone, it might violate all of them. The only hope is if the D&D rules address a situation.

And, as it happens, the rules do address the possibility of a projectile changing size in flight. From the text of spells like Enlarge Person (http://www.d20srd.org/srd/spells/enlargePerson.htm) and Righteous Might (http://www.d20srd.org/srd/spells/righteousMight.htm), we learn that thrown objects do damage according to their size on impact, but that projectiles (launched from a bow, sling, or similar) do damage according to their size at launch. This does leave one wondering how the laws which govern the universe of a D&D world distinguish between an arm and any other launch mechanism, but regardless of how it works, the rules tell us that it does. So for Shrink Item, you just have to make sure that you throw your weaponized pebbles by hand, not with a projectile weapon.

Magic, duh.

Fenix_of_Doom
2007-09-23, 04:02 PM
Enlarge Person would increase falling damage; Reduce Person would decrease it. Or at least, it would, if the D&D rules paid heed to physics and changed falling damage according to size, but they don't.

Really? So if I'd drop a normal sized human of a cliff that will break his bones but not kill him and then drop a giant down the cliff that is just a small step down for him, the giant suffers more damage then the human?

If we are talking during flight size increase/decrease, then according to you, my conclusion in the first post must have been wrong, in any case, please explain yourself.

Dervag
2007-09-23, 05:07 PM
Really? So if I'd drop a normal sized human of a cliff that will break his bones but not kill him and then drop a giant down the cliff that is just a small step down for him, the giant suffers more damage then the human?Actually, yes.

For instance, for a human being, falling over is unlikely to be fatal. For a giraffe, it is. The reason is simple. The strength of parts of the human body increases proportionate to the square of your height- twice as tall means you can carry four times as much load.

But the mass of your body, and therefore the amount of energy released when you hit the ground in a fall, is proportionate to the cube of your height. Twice as tall means you weigh eight times as much, hit the ground with eight times as much kinetic energy from falling the same distance, and have eight times as much force transmitted through your body.

So if you were twice as tall, you would get hit with relatively twice as much punch from the same fall. This is why a cat can survive a long fall that would seriously injure a human or kill a horse.

For a giant, falling over is a much more serious problem and far more likely to cause injury than for a human, unless the giant's bones and muscles are made of materials proportionately stronger than a human's. A twelve-foot giant needs bones and muscles twice as strong as a six-foot man's per unit cross-section.

Now, granted that a giant can handle a ten foot drop more easily than a six-foot man can, but only by using techniques that the man could use to cover a five foot drop (easing himself over the edge and lowering himself to the ground). Even then, the giant has to be stronger than a human (bigger muscles relative to his body size) to accomplish the same feats in that regard.

Ulzgoroth
2007-09-23, 05:21 PM
It is fairly trivially clear that the muscles and bones of things like giants and dragons are much stronger for their cross-section than those of humans. Otherwise, they just don't work at the sizes they go up to.

On the same note, don't go near the wingtips of a dragon in flight. Their wings beat seemingly at much the same rate as those of large birds...which are over an order of magnitude smaller.

horseboy
2007-09-23, 08:17 PM
Trying to get D&D magic consistent with the laws of D&D magic is hopeless.

There's the other problem in this equation.

Mr. Moogle
2007-09-23, 08:39 PM
ACKKK! TO MUCH PHYSICS *little poping sound, face hits keyboard*

Enzario
2007-09-23, 08:47 PM
Trying to get D&D magic consistent with the laws of physics, particularly the conservation laws, is hopeless. For any conservation law you name, I can find a spell or magic item which violates it (well, maybe not any conservation law: I don't think there are any spells which violate Conservation of Strangeness, say, but certainly Mass, Energy, Momentum, and Angular Momentum are out the window). So when we have a spell which clearly violates at least one of the laws, I don't think it's meaningful to try to figure out which one it violates: For all we can tell from physics alone, it might violate all of them. The only hope is if the D&D rules address a situation.

Well, not really, if you figure in the existence of the Inner Planes. According to 'standard' DnD cosmology, there are four primary elemental energy sources, as well as the opposing planes of positive and negative energy. Since each of these six planes is (theoretically) infinite, then it follows that in each of these planes is a correspondingly infinite amount of raw elemental energy. On close inspection of fluff (yeah, that's paradoxical, but so is quantum physics) it would seem that spells derive their energy from elemental sources. So, it seems logical that what spells do is to open up carefully coordinated gateways to these planes and then shape the ensuing energy as to create the desired effect (for a good image of what this means, check out the Wheel of Time series; same basic concept). In that case, the laws of conservation are still unbroken, since the theory in use in our world only takes into account matter within a single, closed universe. Introduce different planes of existence of pure energy, then the mass, energy, etc. already exists, just not on the same plane. Open up a small gateway between, and poof, magic.

Actually, that raises an interesting idea: magic at its most basic form is simply a very controlled wormhole technology. Then what, does permanency create a closed loop of wormholes, keeping the energy in constant circulation? ^^note to self for next world-building project

Chronos
2007-09-23, 11:25 PM
Well, not really, if you figure in the existence of the Inner Planes.Perhaps, but that doesn't really help with the problem at hand. Looked at that way, mass, energy, momentum, and angular momentum can all flow from the elemental/energy planes to the material, but it still doesn't tell us which of those (or which combination) is flowing to power any given spell.

Looked at this way, by the way, permanency must work by keeping the portals open, not just recycling the energy. There are a fair number of Permanency tricks which allow energy to be withdrawn from a system indefinitely. For instance, set up a permanent Antigravity Field, and mount a flywheel on the edge of it (so half the wheel is falling down, and the other half is "falling" up). Absent any force slowing the wheel down, it'll spin faster and faster without limit, and you could attach a generator to it to keep it going at a constant speed, but pumping out energy.

Incidentally, I've personally come to the conclusion that the existence of Spheres of Annihilation is a necessary consequence of the existence of all of these ways for matter and energy to enter the world. Without Spheres, the world would be full of sources without sinks, and could not be in equilibrium. The elemental and energy planes, meanwhile, are probably embedded in anti-deSitter spacetimes, allowing them to remain at a constant density despite the constant drain from the Material.

Dervag
2007-09-23, 11:34 PM
Say, Chronos, have you ever heard of "the Warlock's Wheel?" It reminds me of your idea.

It is fairly trivially clear that the muscles and bones of things like giants and dragons are much stronger for their cross-section than those of humans. Otherwise, they just don't work at the sizes they go up to.You're right. But even so, they are going to get hit harder by a fall of given height, and should suffer a larger percentage of their hit points in damage. Or at least an equal percentage, roughly, which means that fall damage should scale with size.

horseboy
2007-09-23, 11:43 PM
On close inspection of fluff (yeah, that's paradoxical, but so is quantum physics) it would seem that spells derive their energy from elemental sources. So, it seems logical that what spells do is to open up carefully coordinated gateways to these planes and then shape the ensuing energy as to create the desired effect
Especially if you factor in Hawking's theorem about how matter can disappear completely if it's inside a black hole. With a concept of Inner planes it would be used to explain where the matter as coming/going.

Eldritch_Ent
2007-09-23, 11:46 PM
Too much physics makes my head hurt, but I'll put in these two cents-

I agree that shrinking a pebble, dropping it, then expanding it into a boulder *ON IMPACT* Would, at best, cause damage equal to the damage the pebble would have caused by itself, plus The damage from the forces created by the boulder expanding, plus whatever damage having a boulder suddenly sitting on top of you would cause.

If you're going to set a contingency on boulders, I think your best bet would be to lob them upward and have the contingency set on "When the pebble reaches the apex of it's flight", or "Begins falling unattended", then aim so the pebble is mostly over the target, since I figure it would stop moving horizontally since it takes far less energy to throw a pebble than a boulder.

Setting the condition to "When I throw it" might be problematic- (Since you suddenly have a number of boulders with minimal kinetic energy in/on your hand. The expansion alone could probably tear some tendons and break some bones, and if the boulders expanded to cover you.... :smalleek: )

Of course, this would be no problem for one of the ridculously strong Hulking Hurler Builds- What with the carrying capacity to pick up and throw the entire planet if needed, and carrying around a boulder of the densest non-radioactive material they could think of- Namely, Solid, pure Iridium. Of course, bringing him up here would also probably be a bad idea... Oops! :smallbiggrin:

Kaelik
2007-09-24, 12:05 AM
The whole point of the Shrink Item set up is not to have them grow on impact. It is to drop them from a height, and then dismiss the spell, allowing the large boulders that you could not have carried that high up to fall and crush something below. The whole point is that it would be exactly the same as a large boulder falling from the sky. So it doesn't matter if the kinetic energy is conserved. (Note that the total energy could not be conserved anyway since there will be a significant influx of potential energy with the expansion anyway.)

All that matters is that big old boulders are about to crush you. That's it.

Citizen Joe
2007-09-24, 07:34 AM
You're right. But even so, they are going to get hit harder by a fall of given height, and should suffer a larger percentage of their hit points in damage. Or at least an equal percentage, roughly, which means that fall damage should scale with size.

I like to up the damage die for falls for larger creatures, but give them a larger height to fall before taking damage. Basically, you can fall your reach distance and catch yourself, but beyond that you take dN damage for every 10' fallen (up to 200'). So a large creature would take d8 damage, huge d10, gargantuan d12, collossal d20. There are exceptions. Some creatures aren't fleshbags that explode when they hit the ground. Very small creatures would take correspondingly less damage (d4, d3).

Draz74
2007-09-24, 11:41 AM
Fenix - did you honestly hive off a whole new thread just to up the count on your catgirl cull? Are you trying to powerlevel by rampant nekocide or something? :smallwink:

You did? I'm so proud of you. :smallredface:

Chronos
2007-09-24, 12:01 PM
Especially if you factor in Hawking's theorem about how matter can disappear completely if it's inside a black hole.I'm not sure which theorem you're referring to, but it sounds like you're misremembering it. When matter falls into a black hole, the black hole's mass increases, and the only way for the hole's mass to decrease is to radiate away energy equivalent to the mass it loses. There's no question at any point about where the mass goes.

And Dervag, no, I haven't heard of the Warlock's Wheel. Is it a book?

goat
2007-09-24, 12:15 PM
No, he's right. Denser objects are relatively less affected by air resistance. A sphere of iron and a hollow sphere of paper will not fall at the same speed in an atmosphere; the paper sphere will reach terminal velocity much sooner while the sphere of iron will continue to accelerate towards the ground for longer and therefore hit much harder.

I was only referring to his comment about the gravitational increase from dropping a greater mass, not the relative buoyancies of the objects involved.

I think to even start to properly argue the nature of physics in the D&D world, we still need to answer a lot of questions. I think the most important of these would be: do the laws of thermodynamics still apply? With inter-planar travel being relatively easy, it would be difficult to apply any form of energy conservation to a single plane. Applying it to the multi-planar universe as a whole could be more successful, but spells like Genesis and its psionic equivalent seem to create something from nothing, and somewhere from nowhere.

horseboy
2007-09-24, 01:00 PM
I'm not sure which theorem you're referring to, but it sounds like you're misremembering it. When matter falls into a black hole, the black hole's mass increases, and the only way for the hole's mass to decrease is to radiate away energy equivalent to the mass it loses. There's no question at any point about where the mass goes.

According to his theorem, there comes a time in a black hole's life where it "ends". When the black hole "dies" it and all the matter it's collected cease to exist in our reality. It sounded to me like he was trying to say that the universe uses black holes to help "drop" remainders.

Ted_Stryker
2007-09-24, 01:56 PM
According to his theorem, there comes a time in a black hole's life where it "ends". When the black hole "dies" it and all the matter it's collected cease to exist in our reality. It sounded to me like he was trying to say that the universe uses black holes to help "drop" remainders.
I think you're mixing up two different things here.

When matter falls into a black hole, we lose all information about that matter except for its mass, angular momentum, and electric charge.

Hawking radiation (http://en.wikipedia.org/wiki/Hawking_radiation), if it truly exists, is what is emitted by a black hole after it forms. The black hole could evaporate, but there would be an amount of matter and radiation equivalent to its mass that it would have emitted during its lifetime.

horseboy
2007-09-24, 02:46 PM
I think you're mixing up two different things here.

When matter falls into a black hole, we lose all information about that matter except for its mass, angular momentum, and electric charge.

Hawking radiation (http://en.wikipedia.org/wiki/Hawking_radiation), if it truly exists, is what is emitted by a black hole after it forms. The black hole could evaporate, but there would be an amount of matter and radiation equivalent to its mass that it would have emitted during its lifetime.
Not according to the biography they were doing for him on Discovery. He was claiming that the information inside disappeared from reality. The whole "violating the laws of conservation of matter and energy" was the key point of the show. Then at the end of the show, they showed him at the big science convention where he claimed that it didn't disappear, but went to an alternate reality, or some such, and therefore, the laws don't apply to our reality, but to the sum total of all realties.

Chronos
2007-09-24, 06:26 PM
Black holes do conserve mass and energy. They may or may not conserve information (that's one of the biggest and most interesting open questions in the field right now), but that's not the same thing. A few years ago, Hawking made some claims to progress on that score, but they weren't very rigorous, and I think he's since retracted them.

Ted_Stryker
2007-09-24, 08:52 PM
Not according to the biography they were doing for him on Discovery. He was claiming that the information inside disappeared from reality. The whole "violating the laws of conservation of matter and energy" was the key point of the show. Then at the end of the show, they showed him at the big science convention where he claimed that it didn't disappear, but went to an alternate reality, or some such, and therefore, the laws don't apply to our reality, but to the sum total of all realties.
Information isn't the same as matter. In fact, information is essentially an organizational property of matter, and it's not clear whether it's conserved. In fact, in classical general relativity (i.e. GR without quantum mechanics), information is clearly not conserved when you are dealing with black holes. This is the so-called "no hair theorem" (http://en.wikipedia.org/wiki/No_hair_theorem), which asserts that a black hole can be fully characterized by its mass, charge, and angular momentum, irrespective of whatever falls into it. But if you bring quantum mechanics to bear, the issue is still in doubt, and this is one of the things that Hawking has worked on.