More questions for the physics gurus: Sudden mass change
Fitz
object in flight?
Let's take the example of a "reduced" boulder, dispelled during flight
so that it returns to its original size and weight.
Or, to take an opposite example, what about a huge boulder fired from
a trebuchet which is magically shrunk in flight?
Whenever I try to figure out what would happen, my brain turns inside
out and dribbles out my ears.
+++++++++++++++++++++++++++++++++++++
Fitz
http://mojobob.netnet.net.nz
http://fitz.jsr.com
http://usa.spis.co.nz/fitz
+++++++++++++++++++++++++++++++++++++
Ben Sisson
that IS his real name) conspiratorially whispered:
>What effect would a sudden mass change have on the trajectory of an
>object in flight?
Virtually none. An object in free flight is only affected by air
resistance and gravity. The air resistance difference would be
negligable, especially since it is not modelled in D&D. Accelleration
downwards due to gravity doesn't depend on mass (at least as far as
we're concerned here). This is why dropping a ten kilogram weight and
a ten gram weight from the same height will have them hit the floor at
the same time.
>
>Let's take the example of a "reduced" boulder, dispelled during flight
>so that it returns to its original size and weight.
It's trajectory, for all game intents and purposes, would be
unchanged. Neat trick, btw.
>
>Or, to take an opposite example, what about a huge boulder fired from
>a trebuchet which is magically shrunk in flight?
Again, no change in trajectory. Its mass would be a lot less, though.
Would be awfully hard to actually cast on something already in flight;
Reduce's range isn't that good.
>
>Whenever I try to figure out what would happen, my brain turns inside
>out and dribbles out my ears.
What happens is its mass changes, its trajectory does not. Pretty
simple really.
--
Ben Sisson
"Asking the ISU to investigate itself is like asking Enron to do an
internal audit."
- Brian Williams on the Salt Lake Screwjob in figure skating
Hong Ooi
>What effect would a sudden mass change have on the trajectory of an
>object in flight?
>
>Let's take the example of a "reduced" boulder, dispelled during flight
>so that it returns to its original size and weight.
>
>Or, to take an opposite example, what about a huge boulder fired from
>a trebuchet which is magically shrunk in flight?
>
>Whenever I try to figure out what would happen, my brain turns inside
>out and dribbles out my ears.
>
The question is ill-posed, since objects can't change their mass (at least
not without invoking quantum physics). Use whatever seems most dramatically
appropriate to the situation.
--
Hong Ooi | "Usually you're a funny guy Hong. Here
hong...@maths.anu.edu.au | you're just being petty and trollish."
http://www.zipworld.com.au/~hong/dnd/ | -- BWS
Sydney, Australia |
Tom Garnett
> not without invoking quantum physics). Use whatever seems most
dramatically
> appropriate to the situation.
Rocket equations?
Relativistic speeds?
...but yes, that won't happen to the average boulder.
Basically, the trajectory would stay the same, unless more bits of physics
were violated, too.
(You could also rule that total kinetic energy was conserved - which would
cause it to drop out of the sky there, or even that total eberyg was
conserved, which would do something evry very strange, considering the
amount of PE it would suddenly lose.)
Personally, I'm all in favour of the siege-sling option.
--
Tom Garnett
Hong Ooi
>> The question is ill-posed, since objects can't change their mass (at least
>> not without invoking quantum physics). Use whatever seems most
>dramatically
>> appropriate to the situation.
>
>Rocket equations?
>Relativistic speeds?
The ether is real in my world. ;)
Hong "don't ask what colour the sky is" Ooi
Leif Magnar Kj|nn|y
Tom Garnett <tm...@cam.ac.uk> wrote:
>> The question is ill-posed, since objects can't change their mass (at least
>> not without invoking quantum physics). Use whatever seems most
>> dramatically
>> appropriate to the situation.
>
>Rocket equations?
Not really a case of an object changing its mass. More like
one object splitting into several objects, most of which are
thrown out the ass end of the rocket. Some energy goes here,
some goes there; same for momentum, everything is conserved.
>Relativistic speeds?
Not a case of an object changing its mass either, really. Just
looks that way to someone who's been standing still.
>...but yes, that won't happen to the average boulder.
>
>Basically, the trajectory would stay the same, unless more bits of physics
>were violated, too.
I prefer to pretend to use Aristotelian physics in such games, anyway.
"Kinetic energy, wot's dat?" neatly prevents any further difficult
questions regarding that issue.
--
Leif Kj{\o}nn{\o}y | "Its habit of getting up late you'll agree
www.pvv.org/~leifmk| That it carries too far, when I say
Math geek and gamer| That it frequently breakfasts at five-o'clock tea,
GURPS, Harn, CORPS | And dines on the following day." (Carroll)
Tom Garnett
>
> Not a case of an object changing its mass either, really. Just
> looks that way to someone who's been standing still.
Like the target...
(Relativistic projectiles are fun, but far more so in hard(ish) sci-fi than
fantasy...)
--
Tom Garnett
NeoSadist
"Fitz" <pj_...@hotmail.com> wrote in message
news:r8bk6uguvh72q6ejq...@4ax.com...
Yeah, I think it would. I think you'd have to re-roll the attack to see if
it actually hit.
Sir Bob
> On Wed, 13 Feb 2002 22:17:00 +1300, Fitz <pj_...@hotmail.com> wrote:
>
> >What effect would a sudden mass change have on the trajectory of an
> >object in flight?
> >
> >Let's take the example of a "reduced" boulder, dispelled during flight
> >so that it returns to its original size and weight.
> >
> >Or, to take an opposite example, what about a huge boulder fired from
> >a trebuchet which is magically shrunk in flight?
> >
> >Whenever I try to figure out what would happen, my brain turns inside
> >out and dribbles out my ears.
> >
>
> The question is ill-posed, since objects can't change their mass (at least
Magic. Say it with me. Mah-jick. Magic. ;)
- Sir Bob.
P.S. Nih!
Chris
> On Wed, 13 Feb 2002 22:17:00 +1300, Fitz <pj_...@hotmail.com> (if
> that IS his real name) conspiratorially whispered:
>
> >What effect would a sudden mass change have on the trajectory of an
> >object in flight?
>
>
Is this right? Does the change also create a corresponding change in
its kinetic energy? I think maybe Hong Ooi's right, you cannot really
answer the question based on an understanding of physics, as it
doesn't really apply to such a magical change.
Jens Steinhoff
> object in flight?
It depends on how the "laws of physics" work in your world. In
reality objekts don't undergo suden mass-changes, so if you alter
the "laws of physics" to allow that, make it so, that the results
are most appropriate for your taste.
Here is how I' d rule that:
p = mv (impulse equals mass times velocity)
p = const (conservation of impulse)
So if you suddenly double the mass the velocity halves and vice versa
mfg
Jens
Sea Wasp
>
> What effect would a sudden mass change have on the trajectory of an
> object in flight?
>
> Let's take the example of a "reduced" boulder, dispelled during flight
> so that it returns to its original size and weight.
>
> Or, to take an opposite example, what about a huge boulder fired from
> a trebuchet which is magically shrunk in flight?
You have to decide which laws of physics apply:
If Conservation of Momentum applies, then in the first case, it goes
from flying forward at, say, 60 MPH, to flying forward at a miniscule
rate of 60/mass ratio. If the change is at all large, basically the
pebble flies forward and then turns to a boulder and stops dead in
midair, to plummet straight down with a crash. In the second case, the
regular boulder shrinks and suddenly you have a high-velocity bullet,
or even a mini-meteoroid, screaming along the same general vector, but
with vastly increased range due to the speed. (f'rinstance, if the
trebuchet imparted a 60MPH speed to the boulder, and you used a spell
like "Shrink Item" on it, it shrinks to 1/12 the dimensions, which
would be roughly 1/1700 the mass. This would give you [from a boulder
weighing about a ton] a stone, weighing slightly more than a pound,
travelling at 102,000 MPH!!!)
If conservation of energy holds, you'll see similar effects, but much
less extreme (due to the square relationship between velocity and KE)
If these don't hold, but instead there's a constant velocity law in
effect, then the pebble-to-boulder becomes a devastating attack and
the boulder-to-pebble a good defense.
--
Sea Wasp http://www.wizvax.net/seawasp/index.htm
/^\
;;; _Morgantown: The Jason Wood Chronicles_, at
http://www.hyperbooks.com/catalog/20040.html
azothath
> object in flight?
>
> Let's take the example of a "reduced" boulder, dispelled during flight
> so that it returns to its original size and weight.
>
> Or, to take an opposite example, what about a huge boulder fired from
> a trebuchet which is magically shrunk in flight?
>
> Whenever I try to figure out what would happen, my brain turns inside
> out and dribbles out my ears.
>
lol.. well it depends on if the mass shows up with momentum/energy or
not.
Normal physics applies (in a very rough newtonian sense) in D&D3 so
long as magic is not a factor. So just use your common sense for most
common events. Remember that time is now quantized to 6 second
periods, space to about 10cm, and mass to about 20g.
With magic there is a quandry, but the best assumption is that things
generally go along with how they were at the time of casting. Thus if
the boulder is flying when the Enlarge hits, it will continue to go
and the mass will show up with some velocity. If it's a static
boulder(just sitting there) the mass shows up without velocity. This
is an attempt to keep the experience similar at the cost of additional
energy.
If you decide that rules of conservation of energy applies to magic...
there are consequences as basically magic defies general
relativity(thermodynamics) in general. If mass shows up with NO
momentum on a parabolic flight path boulder, the forward momentum is
going to drop and the distance flung shortened. The reverse for the
suddenly shrunk boulder. It gets interesting as if one "shrunk" the
gas in a baloon it might get rather warm. All in all I'd say skip
this option unless you ARE a physics major.
=====
can it be?!
E=mc^2+p^2c^4forflyingbou...@notsohotmail.com.invalid
Reginald Blue
news:cf2f12ef.0202...@posting.google.com...
> Hong Ooi <hong...@maths.anu.edu.au> wrote in message
news:<61kk6ukg3v1gpopjq...@4ax.com>...
> > On Wed, 13 Feb 2002 22:17:00 +1300, Fitz <pj_...@hotmail.com> wrote:
> >
> > >What effect would a sudden mass change have on the trajectory of an
> > >object in flight?
> >
least
> > not without invoking quantum physics). <snip>
>
> Magic. Say it with me. Mah-jick. Magic. ;)
I see the ;) there, but it should be stated that the question is still ill
posed because it's mixing magic and physics. The spell just doesn't say
what happens when you shrink and object in flight so the OP decided that
(real-world) physics must answer the question.
Of course, I see two possible ways answers to this question:
1. Momentum is conserved.
2. Momentum is not conserved.
Oddly, in either case, the trajectory doesn't change, because momentum is a
vector and, while the magnitude of the vector may or may not change, the
direction shouldn't.
But, really, the only real answer is: "What does the spell say?"
(nothing). Then: "What does the GM say and/or what does the Sage say?"
Any answer you'd get is equally valid because, as you say, it's Magic.
David Klassen
>
> >What effect would a sudden mass change have on the trajectory of an
> >object in flight?
> >
> >Let's take the example of a "reduced" boulder, dispelled during flight
> >so that it returns to its original size and weight.
> >
> >Or, to take an opposite example, what about a huge boulder fired from
> >a trebuchet which is magically shrunk in flight?
> >
> >Whenever I try to figure out what would happen, my brain turns inside
> >out and dribbles out my ears.
> >
>
> The question is ill-posed, since objects can't change their mass (at least
> not without invoking quantum physics). Use whatever seems most dramatically
> appropriate to the situation.
Working with only Newtonian physics, yep. No mass in any of the kinematic
equations of motion. Now, there would suddenly be an increase in the
rocks Kinetic and Potential Energies that you may need to address -- but
I think I'd do the easy thing and sweep it under the rug as being created
by the spell energies.
bjm-...@phreeow.net
RB> Of course, I see two possible ways answers to this question:
RB>
RB> 1. Momentum is conserved.
RB> 2. Momentum is not conserved.
RB>
RB> Oddly, in either case, the trajectory doesn't change, because momentum is a
RB> vector and, while the magnitude of the vector may or may not change, the
RB> direction shouldn't.
In a vacuum. In atmosphere the trajectory will certainly change, and
fairly dramatically, both due to drag and wind direction.
--
Brad Murray * Always carry a short length of fibre-optic cable. If
Perl Geek * you get lost, then you can drop it on the ground, wait
VSCA Founder * ten minutes, and ask the backhoe operator how to get
Magnet Oper * back to civilization. (Alan Frame)
Reginald Blue
news:oZxa8.10790$_41.1...@news0.telusplanet.net...
> Reginald Blue <Regina...@hotmail.com> wrote:
> RB> Of course, I see two possible ways answers to this question:
> RB>
> RB> 1. Momentum is conserved.
> RB> 2. Momentum is not conserved.
> RB>
> RB> Oddly, in either case, the trajectory doesn't change, because momentum
is a
> RB> vector and, while the magnitude of the vector may or may not change,
the
> RB> direction shouldn't.
>
> In a vacuum. In atmosphere the trajectory will certainly change, and
> fairly dramatically, both due to drag and wind direction.
Not at the precise instant that the mass changed, but certainly mere moments
thereafter.
How one would determine what that would mean would vary from circumstance to
circumstance. I would arge that shrinking a 10 ton boulder to 1/2 it's size
wouldn't likely result in much change in trajectory. At least not enough to
matter to the guy on the receiving end. :-)
Gorg Huff
>Is this right? Does the change also create a corresponding change in
>its kinetic energy? I think maybe Hong Ooi's right, you cannot really
>answer the question based on an understanding of physics, as it
>doesn't really apply to such a magical change.
Kinetic energy is a function of mass and velocity. If you magically
change the mass. Then either the Kinetic energy increases or the
velocity must decrees to compensate for the increased mass. If there
were a sudden change in velocity associated with the spell it would be
noted. Imagine the effect of casting a spell that more then dubbed
velocity on someone who is walking or turning when the spell is cast.
If however it increased Kinetic energy the change would fit with in
the preconceptions of the time and not be noted. "Big rocks hit harder
then little rocks. This is news?"
So unless there are notes specifying a change in velocity, and
there aren't in either spell mentioned, the reasonable assumption is
that the change in size/mass does not effect velocity but does effect
Kinetic Energy. In fact the part of the description that deals with
enlarged rocks hitting harder would at least tend to support that
assumption.
Gorg
Gorg Huff
<Yog-S...@demons-den.com> wrote:
>Here is how I' d rule that:
>p = mv (impulse equals mass times velocity)
>p = const (conservation of impulse)
>So if you suddenly double the mass the velocity halves and vice versa
So in your universe if you cast Reduce on someone in your universe
just as they make a leap Their velocity doubles as their mass halves.
So they jump 4 * as far?
Gorg
Martin Leslie Leuschen
: Oddly, in either case, the trajectory doesn't change, because momentum is a
: vector and, while the magnitude of the vector may or may not change, the
: direction shouldn't.
The trajectory depends on the acceleration do to gravity as well,
and the vectors add, so if momentum is conserved, the trajectory
*will* change.
In any case, the answer to the shrunk/enlarged boulder question
is arbitrary - the spell seems to violate conservation of
mass-energy, so normal physics does not apply.
GM's judgment tells me it's simpler if things just keep going
on on their old trajectory, though.
Regards,
martinl
Grey
> object in flight?
>
> Let's take the example of a "reduced" boulder, dispelled during flight
> so that it returns to its original size and weight.
>
> Or, to take an opposite example, what about a huge boulder fired from
> a trebuchet which is magically shrunk in flight?
>
> Whenever I try to figure out what would happen, my brain turns inside
> out and dribbles out my ears.
Going strictly by the laws of physics, I'd rule that the total kinetic
energy would probably stay the same and therefore it would greatly
slow down.
BUT, physics can't really account for a sudden change in mass. Well,
it probably could, but it would be way more trouble than it's worth.
Basically, I would rule that since it's a magical change, it's not
subject to the laws of physics and therefore it would keep it's same
trajectory and speed, just with greatly increased mass.
Loren Pechtel
wrote:
>> >Relativistic speeds?
Or a very strange universe. Consider the story "The Redshift
Rendevous" (I hope I spelled that right!). The author is John, I
can't remember his last name. Stith, perhaps?
Most of the story takes place on a starship in a different
universe where lightspeed is only 10 meters per second. Relativistic
effects show up all over the place. I've been toying with the idea of
sending a party to such a universe someday. (However, I don't think
his physics works. I think it will take as much energy to accelerate
an object to x% of c no matter what c is.)
Loren Davis
"Sea Wasp" wrote:
> Fitz wrote:
> >
> > What effect would a sudden mass change have on the trajectory of an
> > object in flight?
> You have to decide which laws of physics apply:
I agree, but there's one wrinkle no one's brought up yet.
> If Conservation of Momentum applies, then in the first case, it goes
> from flying forward at, say, 60 MPH, to flying forward at a miniscule
> rate of 60/mass ratio.
But this same ratio would have to hold in every frame of reference,
which is impossible. Let's say that one person observes that its velocity
decreases from 60 MPH to 20 MPH, a factor of 3. Someone riding at 10 MPH in
the same direction as the boulder will observe a decrease from 50 MPH to 10
MPH, a factor of 5. Someone riding in the opposite direction at 10 MPH will
observe a decrease from 70 MPH to 30 MPH, a factor of 2 1/3. More generally,
a change in velocity cannot conserve an object's momentum if its mass is not
also conserved. Relativity makes the situation more complicated, but we
still get a contradiction if we try this.
> If conservation of energy holds, you'll see similar effects, but much
> less extreme (due to the square relationship between velocity and KE)
The analogous argument also holds.
> If these don't hold, but instead there's a constant velocity law in
> effect, then the pebble-to-boulder becomes a devastating attack and
> the boulder-to-pebble a good defense.
I agree.
CSR
Fitz wrote in message ...
>What effect would a sudden mass change have on the trajectory of an
>object in flight?
>
>Let's take the example of a "reduced" boulder, dispelled during flight
>so that it returns to its original size and weight.
>
>Or, to take an opposite example, what about a huge boulder fired from
>a trebuchet which is magically shrunk in flight?
>
>Whenever I try to figure out what would happen, my brain turns inside
>out and dribbles out my ears.
>
Well all things fall at the same rate regardless of mass ( well, until you
get big enough that you are talking planets attracting each other )
So, in a vacuum, nothing would happen ( remember the apollo astronaut
droping the eagle feather and the rock hammer on the moon and they both hit
the ground at the same time - no air resistance )
However, your missile is in an atmosphere, therefore subject to air
resistance. This can be thought of as a force acting against the objects
momentum to slow it down.
If the object is the same size, it's air resistance stays the same. If it's
mass drops, it's momentum does as well ( easier to stop a ball of styrofoam
than a connon ball of the same size )
So the mass reduced object slows down. Maybe really fast.
Now if you shrink it but keep the mass the same, you have the opposite
effect.
So, if the ratio of mass to size increases, the object flies farther. If the
size relative to mass increases, the object falls short of where it would
have.
CSR
Jacob Frey
On the case of pebble to boulder:
If the mass changes but the size stays the same
If velocity before = velocity after
results in same trajectory, hits harder.
If momentum before = momentum after
results in reduced speed, hits equally as hard.
If the mass changes and the size changes
results in increased air resistance, reduced range.
If you use a liberal application of Einstein's E=mc^2, then energy can
be changed into mass.
result is mass increased, overall energy decreased by great amounts.
Effectively E/c^2 where c is the speed of light [3x10^8 m/s] <and dont
forget to square it!>
eh... to complicated, it is not physics, but magic. IMO same traj. hits
harder.
I just thought of something. If there is an NPC who tosses a coin in
the air and catches it, or a group of jugglers, this use of the spell
could be life threatening. (but hilarious)
Kaos
>What effect would a sudden mass change have on the trajectory of an
>object in flight?
>
>Let's take the example of a "reduced" boulder, dispelled during flight
>so that it returns to its original size and weight.
>
>Or, to take an opposite example, what about a huge boulder fired from
>a trebuchet which is magically shrunk in flight?
No effect.
<handwave>It's magic</handwave>
--
This is usenet.
Truth is secondary to Presentation.
Jonathan Bristow
to
> circumstance. I would arge that shrinking a 10 ton boulder to 1/2 it's
size
> wouldn't likely result in much change in trajectory. At least not enough
to
> matter to the guy on the receiving end. :-)
>
In a vacuum, shrinking a 10 ton boulder at the apex of it's flight would not
diminish the amount of time it would take for it to reach the ground,
HOWEVER, momentum must be conserved... and half the weight would equal twice
the horizontal speed, thus increasing your range by 50% (since we're
measuring from apex)... conversely, twice the weight would equal half the
speed, reducing your range by 25%
Christopher Burke
news:a4fv2p$ftd$1...@news1.ucsd.edu:
Bugger - does that mean throwing that magically miniturised 50 ton rock at
the beholder wouldn't work ... cause as soon as it got heavy in the anti-
magic eye it would stop dead in its trajectory ?
--
---
/* Christopher Burke - Spam Mail to cra...@hotmail.com
|* www.craznar.com -
\* Real mail to cburke(at)craznar(dot)com
tussock
>
> What effect would a sudden mass change have on the trajectory of an
> object in flight?
An extremely large explosion, object ceases to exist, as do nearby
planets. Unless you're increasing mass, then you've just got to suck all
the energy out of everything to produce the change, end of all life at
least.
> Let's take the example of a "reduced" boulder, dispelled during flight
> so that it returns to its original size and weight.
>
> Or, to take an opposite example, what about a huge boulder fired from
> a trebuchet which is magically shrunk in flight?
>
> Whenever I try to figure out what would happen, my brain turns inside
> out and dribbles out my ears.
Your examples simply change mass, so ignore conservation laws, if
the spell breaks conservation of energy through mass change it can break
the rest too.
Double mass then gives double momentum and double knietic energy, or
for most purposes about double damage (less if using big changes, so as
to maintain game balance, try a square root, 1/4 mass = 1/2 damage).
--
tussock
Still need a new .sig
Jens Steinhoff
> just as they make a leap Their velocity doubles as their mass halves.
Yes
> So they jump 4 * as far
Yes L = (v^2*sin(2alpha))/g, if you double the velocity you jump
4 times as far.
That is way I'd rule it, when in my universe were a spell, that is
able to invoke sudden changes in mass of moving objects. But if
you don't like it, make the "laws of magic" so that happens what-
ever you want to happen. As long as they are consistent, make
them do whatever you want (thats why its called magic).
Jens
Gorg Huff
<cra...@hotmail.com> wrote:
>Bugger - does that mean throwing that magically miniturised 50 ton rock at
>the beholder wouldn't work ... cause as soon as it got heavy in the anti-
>magic eye it would stop dead in its trajectory ?
To me that sounds like an amazingly workable plan. Assuming you can
miniaturize a 50 ton rock. 50 tons = 100,000 lb using shrink item 50
tons becomes 50lb. At 20th level your max is 40 square feet translated
into 40 square inches. But a 2.7ft by 2.7ft by 2.7 rock is probably
not going to more then five tons. five lb after it's shrunk. A bit
larger and heavier then a base ball. On the other hand a baseball or a
rock that size can be thrown fairly hard call it fifty miles an hour.
And a five ton rock traveling at fifty miles an hour should be quite
enough to turn a beholder into a behelder.
On the other hand if your DM determines that won't work. That
conservation of kinetic energy rather then velocity occurs. Then you
can cast Enlarge on a two inch rock making it into a three inch rock
that weighs 240% of it's original weight. Throw it at a speed of 50
mph when it hits the field it becomes a 2 inch rock that weighs it's
original weight but is traveling at 120 mph The beholder is unlikely
to find this pleasant either.
In either case what is happening is that your using magic to put
something in the size range to allow you to use your strength on it
most effectively and the loss of that magic to either multiply the
force or concentrate the force in such a way as to do more damage.
Here is an experiment to do in your game world to see which one
works for you.
Go out in a field some where and collect several small rocks. Cast
enlarge on them and toss them gently at a target. Then dispel it while
they are in the air.
If they suddenly shoot off a higher speed, then you have
conservation kinetic energy. You want to have some undersized crossbow
bolts made up to use against Folks with anti magic fields up. So you
can cast enlarge on them before firing. And either a dispell while
they are in flight or running into an anti magic field will turn them
in to something about the size of a bullet and traveling almost as
fast as one. These bolts will hit with the same force as non spelled
bolts but because they are smaller the force will not be as spread out
and will be more likely to penetrate armor of whatever sort.
Effectively they are keen.
On the other hand if they keep going at pretty much the same speed
they probably work under the Conservation of velocity rule. You will
want to do another experiment to confirm that. Pick a larger stone and
cast reduce on it. Toss it at a target and dispell it after it is in
the air. If it hits the target with more force the it would have had
in it's reduced state your good to go. Make up some larger bolts for
your crossbow.
If neither of these works then your best bet is to find your self a
game with an honest DM.
Gorg
Gorg Huff
<Yog-S...@demons-den.com> wrote:
>Yes L = (v^2*sin(2alpha))/g, if you double the velocity you jump
>4 times as far.
Neat. The posibilities are fun.
>That is way I'd rule it, when in my universe were a spell, that is
>able to invoke sudden changes in mass of moving objects. But if
>you don't like it, make the "laws of magic" so that happens what-
>ever you want to happen. As long as they are consistent, make
>them do whatever you want (thats why its called magic).
I would enjoy playing in your game. I can deal with whatever rules as
long as the are consistent. and not too self contradictory.
Gorg
Reginald Blue
news:a4fv2p$ftd$1...@news1.ucsd.edu...
You got me. In the "momentum conserved" case, it does change the
trajectory. My mistake.
Christopher
news:a9e8322d.02021...@posting.google.com...
> Ben Sisson <ilkhanik...@DIESPAMyahoo.ca> wrote in message
news:<1fhk6u4vtohsupa4p...@4ax.com>...
> > On Wed, 13 Feb 2002 22:17:00 +1300, Fitz <pj_...@hotmail.com> (if
> > that IS his real name) conspiratorially whispered:
> >
> > >What effect would a sudden mass change have on the trajectory of an
> > >object in flight?
> >
> >
> > Again, no change in trajectory. Its mass would be a lot less, though.
>
> Is this right? Does the change also create a corresponding change in
> its kinetic energy?
Yes, the kinetic energy is increased or decreased when the size of the rock
is increased or decreased, respectively. There is no change in velocity or
trajectory. However, due to the change in mass, and thus a change in
kinetic energy carried by the rock, the rock will do more or less damage
depending on whether it was enlarged or reduced, again respectively.
-Chris
Chris
> On 13 Feb 2002 08:01:40 -0800, c_s...@hotmail.com (Chris) wrote:
>
> >Is this right? Does the change also create a corresponding change in
> >its kinetic energy? I think maybe Hong Ooi's right, you cannot really
> >answer the question based on an understanding of physics, as it
> >doesn't really apply to such a magical change.
>
Indeed.
> So unless there are notes specifying a change in velocity, and
> there aren't in either spell mentioned, the reasonable assumption is
> that the change in size/mass does not effect velocity but does effect
> Kinetic Energy.
The reasonable assumption regarding the change in two variables is to
favor one over the other? I wouldn't agree with that even if we were
discussing hard core sci-fi (which we aren't). If you are worried
about creating instant catapaults with a pea-shooter, the reasonable
assumption is that kinetic energy is conserved. If you like the
effect, kinetic energy is proportionately changed.
Gorg Huff
>> So unless there are notes specifying a change in velocity, and
>> there aren't in either spell mentioned, the reasonable assumption is
>> that the change in size/mass does not effect velocity but does effect
>> Kinetic Energy.
On 14 Feb 2002 08:13:38 -0800, c_s...@hotmail.com (Chris) wrote:
>The reasonable assumption regarding the change in two variables is to
>favor one over the other? I wouldn't agree with that even if we were
>discussing hard core sci-fi (which we aren't). If you are worried
>about creating instant catapaults with a pea-shooter, the reasonable
>assumption is that kinetic energy is conserved. If you like the
>effect, kinetic energy is proportionately changed.
The reasonable assumption to me. Is that an obvious change: One
that looks unnatural and would therefor be noticed and noted in the
spell description but is not is probably not there. Therefor given a
change in velocity would look decidedly unnatural but a change in
kinetic energy/force would seem natural. Not surprising might well not
be noted.
Further the spell description for enlarge Says "a hurled stone
would have more mass (and cause more damage)" It does not specify that
the stone was enlarged before or after it is hurled and is therefor
not conclusive in supporting the velocity constant view but it is at
least suggestive. Especially when the spell description does not say
anything about change in velocity.
To me this makes the constant velocity interpretation make more
sense. The reasonable interpretation. Or I should say the most
reasonable interpretation. I would not object to a change in velocity
interpretation. I would be put out were such an interpretation to be
sprung on the caster mid game. But not too put out because others are
going to interpret how magic works differently then I. As a DM I would
interpret it as velocity constant. But if a player were to use the
spell in a way that made it clear that they had interpreted it force
constant. I would inform them and let them back up and rethink the
action because their character would know that it did not work that
way in my world.
Gorg
Chris
> > Is this right? Does the change also create a corresponding change in
> > its kinetic energy?
>
> Yes, the kinetic energy is increased or decreased when the size
minor nit, "mass"
> of the rock
> is increased or decreased, respectively. There is no change in velocity or
> trajectory.
Why no change in velocity? Is that stated in the spell, or are you
presuming this (IDHTBIFOM)? I would have thought the other variable
would change. Enlarge a pebble and you get a boulder that drops
(which in and of itself is a neat effect) as opposed to having a sling
shot for a siege engine.
Christopher
No, you're right. By pure "real world" physics, you would have to consider
conservation of energy. In a fantasy game, where the change in mass is
brought on by magic, I think the "conservation of velocity" law of
fantasy-pseudo-physics is easier to work with. Afterall, it's not just
thrown rocks that could be enlarged or reduced.
Suppose a giant eagle was reduced mid-flight? Would it's velocity suddenly
double or triple or more? If so, might the shrunken bird just rip apart?
What if the giant eagle was a polymorphed wizard and he used this "change
size, change velocity" mechanic to achieve speeds far beyond what is
possible for the creatures he polymorphs into? Does that consitute rules
abuse?
Originally, I didn't consider all of the relative laws of physics (that I
knew anyway). Now, I'm just considering gameplay. I think game play wins
out, and "conservation of velocity" is the best way to go. And, in the case
of the big bird wizard polymorphing into the little bird wizard, if the
little bird's SPEED is less than the big bird's, I'd rule that the next
"move" is made at the new bird's speed (thus, there is no round in which the
little bird may move at an over-accellerated rate).
Hope that made sense....
-Chris.
Loren Davis
"Jens Steinhoff" wrote:
> "Gorg Huff" wrote:
> > So in your universe if you cast Reduce on someone in your universe
> > just as they make a leap Their velocity doubles as their mass halves
> Yes
> > So they jump 4 * as far
> Yes L = (v^2*sin(2alpha))/g if you double the velocity you jump
> 4 times as far.
> That is way I'd rule it, when in my universe were a spell that is
> able to invoke sudden changes in mass of moving objects. But if
> you don't like it, make the "laws of magic" so that happens what-
> ever you want to happen. As long as they are consistent, make
> them do whatever you want (thats why its called magic).
This is not consistent, however. You can't get rid of the conservation
of mass and keep the conservation of kinetic energy. Let's add a bit to Gorg
Huff's example.
Our jumper, whom we'll call Johann, is trying to catch a thief named
Heinrich, who's speeding away on a flying carpet. Heinrich, just to rub it
in, is bobbing along only a few cm ahead of Johann. In their frame of
reference, both of them have zero velocity and no kinetic energy. They could
rub noses if they wanted to, and neither one would get hurt.
Right then, Johann gets reduced by the wizard he left behind. According
to your ruling, his velocity "doubles." Everyone's velocity as measured by
himself is always zero, so unless you specify some other frame of reference,
that would mean that Johann's velocity doesn't change. However, you also
state that he jumps four times the distance as before, so you must have
meant that he overtakes Heinrich. This means that Johann's kinetic energy
and momentum, as measured by Heinrich, increase: Johann's boot is now in
Heinrich's back, kicking the thief off his flying carpet. Reduce and enlarge
still turn small projectiles into deadly weapons, which is exactly what you
wanted to avoid. What would have happened if Heinrich had immediately waved
his wand of enlargement at Johann is another question.
Trying to keep our fantasy worlds somewhat recognizable is a good idea,
but we need to consider all the consequences of the changes we make. The
"conserves kinetic energy" or "conserves momentum" rules, which give
different results, also can only work in one frame of reference which no one
has yet specified. The constant-velocity interpretation works just as well,
and is simpler.
Gorg Huff
wrote:
> Trying to keep our fantasy worlds somewhat recognizable is a good idea,
>but we need to consider all the consequences of the changes we make. The
>"conserves kinetic energy" or "conserves momentum" rules, which give
>different results, also can only work in one frame of reference which no one
>has yet specified. The constant-velocity interpretation works just as well,
>and is simpler.
Well in defense of those supporting the varying velocity
interpretations. It always seemed clear to me that their intended
frame of reference was the world. It's possible I was wrong about that
but I don't think so.
I did like your notion of Johann on the flying carpet though. If I
followed the varying velocity notions correctly Johann is now
traveling twice the velocity of the flying carpet. For a little while.
;-)
Another one would be some one on a horse. Take one heck of a ride
check if the horse didn't shrink too. LOL
Gorg
Loren Pechtel
wrote:
> If they suddenly shoot off a higher speed, then you have
>conservation kinetic energy. You want to have some undersized crossbow
>bolts made up to use against Folks with anti magic fields up. So you
>can cast enlarge on them before firing. And either a dispell while
>they are in flight or running into an anti magic field will turn them
>in to something about the size of a bullet and traveling almost as
>fast as one. These bolts will hit with the same force as non spelled
>bolts but because they are smaller the force will not be as spread out
>and will be more likely to penetrate armor of whatever sort.
>Effectively they are keen.
Why not metamagic the enlarge up a few times first? You'll get
things that go right through a dragon.
Chris
<snip example>
> Originally, I didn't consider all of the relative laws of physics (that I
> knew anyway). Now, I'm just considering gameplay. I think game play wins
> out, and "conservation of velocity" is the best way to go.
I didn't think of it like that, but it makes more sense. Also, from a
practical perspective, my concern of having slingshot shooting
catapaults isn't really valid for low level spells, and at higher
levels it isn't that big of a deal.
> And, in the case
> of the big bird wizard polymorphing into the little bird wizard, if the
> little bird's SPEED is less than the big bird's, I'd rule that the next
> "move" is made at the new bird's speed (thus, there is no round in which the
> little bird may move at an over-accellerated rate).
>
> Hope that made sense....
>
It did, and I changed my mind. Thanks.
Jens Steinhoff
conservation
> of mass and keep the conservation of kinetic energy.
I can, it's my world and magic works the way I want it to work. With
consistency I meant, that the effect has to be the same every time the
same type of spell is cast and the other parameters are the same too.
The effect of a spell should be predictable for a knowledgeable (sp?)
magic-user.
> Right then, Johann gets reduced by the wizard he left behind.
According
> to your ruling, his velocity "doubles." Everyone's velocity as
measured by
> himself is always zero,
of course -> Gallileo transformation.
> that would mean that Johann's velocity doesn't change. However, you
also
> state that he jumps four times the distance as before, so you must
have
> meant that he overtakes Heinrich.
Yes E = (1/2)mv^2
>This means that Johann's kinetic energy
Yes
>and momentum, increase
No p = mv
> Johann's boot is now in Heinrich's back
Yes
>kicking the thief off his flying carpet.
Maybe
> Reduce and enlarge still turn small projectiles into deadly weapons,
> which is exactly what you wanted to avoid.
Yes, but less than in a universe with a conservation of speed. If I take
the biggest stone I'm able to throw, let's say 1kg, throw it with a
speed
of 22 m/s (that would make it travel about 50m, if I throw with (in?) an
angle of 45°). Than somebody casts Reduce1000 on this stone, the
result is a 1g "stone" (more a SiO-bullet) that travels with 22000m/s,
very deadly projectile if it hits somebody. But I think it'd be very
difficult to hit somebody with that kind of projectile.
On the other hand, if somebody throws the 1kg stone against the walls of
a castle an somebody casts Increase1000 on it, you'd have a 1 ton
boulder,
traveling with 0,022 m/s, doubtfull that that will reach the walls (or
if that it
does damage it)
My model makes you able to "build" antipersonal weapons (which, if I
am GM, are useless, because of the difficulties in aiming them because
of
the trajectory change in midflight), but prevents "insta-siege-engines".
One question (I don't play dnd, so I dont know the spells reduce or
enlarge):
Do these spells change the mass and the dimensions of an object or is
the
volume constant and you get an increase in density. (A stone sized 1g
stone
would much more be affected by friction and wind than a grain sized 1g
stone)
mfg
Jens
Jens Steinhoff
> check if the horse didn't shrink too. LOL
Thats a good argument against my ruling, an reduce-type spell would make
a very effective anti-cavalry weapon. Make the spell so, that it doesn't
work
on living beings or unwilling targets.
mfg
Jens
Gorg Huff
<Yog-S...@demons-den.com> wrote:
>Thats a good argument against my ruling, an reduce-type spell would make
>a very effective anti-cavalry weapon. Make the spell so, that it doesn't
>work
>on living beings or unwilling targets.
>
>
>mfg
>Jens
>
Unwilling targets get a save to avoid the effect acroding to the
books.
But why is it a good argument against your ruling? I had no problem
with your ruling. Before the last couple of posts. Though I would have
ruled differently for simplicity sake. Now I am starting to. You seem
to be saying, and I could be misinterpreting here, that the reason for
you interpretation is to make the spell less effective. Perhaps even
to discourage original or creative thought in you campaign.
Gorg
Jens Steinhoff
> books.
> But why is it a good argument against your ruling?
I don't know the dnd rules anymore (last time I played dnd was about
15 years ago) and maybe the rules have changed a bit since then ;-).
>I had no problem with your ruling. Before the last couple of posts.
Thanks
> You seem
> to be saying, and I could be misinterpreting here, that the reason for
> you interpretation is to make the spell less effective.
No, definitly not.
It depends on the level of the spell: If it's a low level spell and able
to wipe out whole cavalry regiments then it should be made less
powerfull.
If it's a high level spell that's ok with me.
>Perhaps even
> to discourage original or creative thought in you campaign.
No, of course not. What I don't want is loopholes in the rules, that
players can exploit and would unbalance the campagin. If a 1st level
magic-user could drastically increase the mass of items, magic-users
would most propaby rule the world, ever fought a magic-user, when
your plate armour weighs 10000 pounds and he tries to use a 1000
pound longsword, ever had money problems, when you could multiply
your gold, imagine a magic user who, buys a gem with 20 carat for
20 GP, abracadabra, sells a 24 carat gem (I don't think a trader would
notice the discrepancy between size and weight, assuming the spell
increases the density and not mass and volume), for 24 GP, repeat.
What I do want is a campaign, where the gameplay is balanced and
the world is consistent.
mfg
Jens
Christopher
Thought about this some more. I think I changed my mind.
You could apply a simplification of the laws of conservation to preserve the
game-world's physics using the existing D20 rules. There are rules for
thrown weapons, their sizes, and range increments (don't know them by heart,
but they're there). If a stone is increased to a "big rock" in mid-flight,
I would recheck the range increment and apply any range new penalties to the
original attack roll. Thus, if the rock is made "really" big, and the
attack roll was "barely" sufficient to hit, then the penalties would cause a
miss.
"The rock grew to twice its size and mass, causing a decrease in velocity
due to conservation of momentum, and thus dropped from the sky just before
reaching the ogre's head."
This method of resolution applies normal game mechanics, and is pretty
simple.
-Chris.
Loren Davis
"Jens Steinhoff" wrote:
You snipped attributions again. Please don't. I'll have more to say
about this later.
> > This is not consistent, however. You can't get rid of the
> > conservation of mass and keep the conservation of kinetic energy.
> I can, it's my world and magic works the way I want it to work. With
> consistency I meant, that the effect has to be the same every time the
> same type of spell is cast and the other parameters are the same too.
Redefine "consistency" however you like. It is logically impossible for
any interactions in which mass is not conserved to conserve momentum or
kinetic energy in every inertial reference frame, if non-magical
interactions resemble those in our world. I could expand that sentence to
three pages if you still want to play semantic games, but haven't we seen
enough of that in the Wish threads?
To repeat: the "conservation of energy" ruling does not, in fact,
conserve energy. The "conservation of momentum" ruling does not, in fact,
conserve momentum. The "conservation of velocity" ruling is more balanced,
and also simpler.
> > Right then, Johann gets reduced by the wizard he left behind.
> > According to your ruling, his velocity "doubles." Everyone's velocity as
> > measured by himself is always zero,
> of course -> Gallileo transformation.
I don't see your point.
A Galileo transformation is exactly what I did in my response to Sea
Wasp earlier in this thread, when I showed that the attempt to conserve
momentum fails to actually conserve momentum in other reference frames. You
get the same results with a Lorentz transformation, but the algebra is more
complicated.
This example does not involve a Galileo transformation, because you
never defined the frame of reference in which Johann's velocity "doubles." I
stay within a single frame of reference, Heinrich's. It's enough to know
that Johann's velocity increases in this reference frame to prove that
kinetic energy and momentum are not conserved. His initial kinetic energy
and momentum were zero; his final kinetic energy and momentum are greater
than zero; Q.E.D.
> > However, you also state that he jumps four times the distance as before,
so you must
> > have meant that he overtakes Heinrich.
> Yes E = (1/2)mv^2
Which proves my case. In this frame of reference, Johann and Heinrich's
initial velocities were 0. E was therefore 0. "He overtakes Heinrich" can
only mean that v' > 0. Since m' > 0, E' > 0. Kinetic energy is not
conserved.
> > This means that Johann's kinetic energy
> Yes
> >and momentum, increase
> No p = mv
First: considering the problems you've had so far in communicating,
it's possible that you didn't realize how improper this editing was. I
actually wrote: "This means that Johann's kinetic energy and momentum, as
measured by Heinrich, increase ..." The strawman argument you were
responding to doesn't even make any grammatical sense. Because you removed
the attribution line from your reply, it wasn't even clear from your message
whom you were quoting. I'll give you the benefit of the doubt, but I'd
strongly advise you to learn how to quote correctly before these bad habits
get you into trouble.
Second: v = 0, v' > 0, and m' > 0. Therefore, p = 0, p' > 0, and p' >
p. According to the equation you've just cited, momentum increases.
> > Reduce and enlarge still turn small projectiles into deadly weapons,
> > which is exactly what you wanted to avoid.
> Yes, but less than in a universe with a conservation of speed. If I take
> the biggest stone I'm able to throw, let's say 1kg, throw it with a
> speed of 22 m/s (that would make it travel about 50m, if I throw with
(in?) an
> angle of 45°).
Either preposition is correct. :)
> Than somebody casts Reduce1000 on this stone, the
> result is a 1g "stone" (more a SiO-bullet) that travels with 22000m/s,
> very deadly projectile if it hits somebody. But I think it'd be very
> difficult to hit somebody with that kind of projectile.
Kinetic energy is proportional to the square of velocity. It increases
by a factor of a thousand, the same as enlargement and constant velocity
would give you, and would be easy to aim at the castle wall if it were
thrown in a mostly-flat trajectory. Of course enlarging a small rock to use
as a projectile becomes pointless; so what? You've just exchanged one trick
for another.
> My model makes you able to "build" antipersonal weapons (which, if I
> am GM, are useless, because of the difficulties in aiming them because
> of the trajectory change in midflight), but prevents
"insta-siege-engines".
It prevents any siege engines, actually: the castle's wizard enlarges
the boulder, guaranteeing that it misses. This would work just as well on
any other ranged weapon, making arrows of slaying useless. If anything, the
constant-velocity version of this trick is less powerful, since you can't
use it with true strike to send a tree trunk through an arrow slit, reduce
the target's reaction time, increase the range of a bow by a factor of four,
or shoot something that can outfly a normal arrow. You also can't use these
spells on moving objects to cause collisions, and you save yourself major
headaches if your players ever get on a moving boat or flying castle.
Why is it worth all this, considering that it isn't either more
realistic or more balanced?
> One question (I don't play dnd, so I dont know the spells reduce or
> enlarge): Do these spells change the mass and the dimensions of an
> object or is the volume constant and you get an increase in density.
> (A stone sized 1g stone would much more be affected by friction and
> wind than a grain sized 1g stone)
Surprised to hear it. Yes, these spells change both volume and mass.
You could make that the basis for a house rule that enlarged projectiles
lose accuracy if you wanted.
Thomas Fleming
<cpar...@X.com, where X is hcl> wrote:
>You could apply a simplification of the laws of conservation to preserve the
>game-world's physics using the existing D20 rules. There are rules for
>thrown weapons, their sizes, and range increments (don't know them by heart,
>but they're there). If a stone is increased to a "big rock" in mid-flight,
>I would recheck the range increment and apply any range new penalties to the
>original attack roll. Thus, if the rock is made "really" big, and the
>attack roll was "barely" sufficient to hit, then the penalties would cause a
>miss.
If you're going to insist on enforcing conservation laws, enforce them all.
The extra mass in your enlarged projectile isn't created; it comes from
someplace else. However, in order to preserve conservation or momentum
and energy (and avoid all those relativistic problems that another poster
brought up) it transfers extra momentum and energy from someplace else
in the universe as well. After all, if mass can be non-locally conserved
through the spell, so too can momentum and energy.
The result would be a constant velocity rule, and a preservation of both
real world physics and the known spell effects, without having to make
any new rules for gameplay.
Personally, I think that using real world physics is a mistake in a
setting where earth, air, fire, and water are the only four entries on
the periodic table. D&D physics only looks like real world physics in
a superficial way.
-Thomas Fleming
Department of Physics and Astronomy
Clemson University
(864)656-1592
Gorg Huff
<Yog-S...@demons-den.com> wrote:
>I don't know the dnd rules anymore (last time I played dnd was about
>15 years ago) and maybe the rules have changed a bit since then ;-).
>
Three spells involved: Reduce and Enlarge 1st level change of 10%
per caster level to a max of 50% cast by a 5th level caster. They
don't stack. And can be cast on a single individual or a single
object. No larger then 10 cubic ft. per caster level. max change of
volume and mass is 240% enlarged or 90% reduce. So a 100 lb. rock
could be reduced to 10lb or enlarged 240 lb. By a 5th level caster.
The third spell: Shrink Item can only be cast on an object no more
then 2 cubic ft. per caster level but reduces the object to 1/12 size
about 1/2000th the mass and volume. Needs a 5th level caster to cast
it at all. A 5th level caster can cast it only on an object 10cu.ft.
or smaller in size. It's also relatively unstable. Tossing it on the
ground can despell it. Depending on dm interpretation using a catapult
or other throwing device might fail.
Gorg
David Klassen
> You have to decide which laws of physics apply:
>
> If Conservation of Momentum applies,...
> If conservation of energy holds,...
I don't see where either of these must be true. The spell itself is
*creating* mass and instantly adding it to the object. There is no
interaction that is free of outside interaction, thus neither conservation
law is applicable. The former is *only* true if there is no outside force
acting on the system, the latter is *only* true if there is no energy
source/sink outside the system. The effect of the spell is pretty much
"outside the system". Thus, neither need be true.
> If these don't hold, but instead there's a constant velocity law in
Constant velocity isn't a law here. It is merely the consequence of the
kinematics of the problem. Position as a function of time is not dependant
upon mass so long as there are no in-system interactions.
Sea Wasp
>
> Sea Wasp <sea...@wizvax.net> wrote in message news:<3C6A9B...@wizvax.net>...
> > You have to decide which laws of physics apply:
> >
> > If Conservation of Momentum applies,...
> > If conservation of energy holds,...
>
> I don't see where either of these must be true.
Um, there's a word. "If". You may notice that I use this word above.
This word implies that we are considering possible alternative
versions of the problem. "If" generally does not mean "this must be
true", at least not in the version of English that exists in the
universe I thought I was posting to.
Since we do not know which version of physics, if any, the original
person was using in his world, we must consider ones in which CoM and
CoE hold, versus ones in which they do not.
The spell itself is
> *creating* mass and instantly adding it to the object.
This is, of course, one way that you can view it. It is not the only
way.
> > If these don't hold, but instead there's a constant velocity law in
>
> Constant velocity isn't a law here. It is merely the consequence of the
> kinematics of the problem. Position as a function of time is not dependant
> upon mass so long as there are no in-system interactions.
Er, seems to me you'd have to have ONE of the above choices, or else
say "it does whatever amuses the GM the most at the moment" and have
done with it, which is a particularly UNhelpful answer to the question
posed.
--
Sea Wasp http://www.wizvax.net/seawasp/index.htm
/^\
;;; _Morgantown: The Jason Wood Chronicles_, at
http://www.hyperbooks.com/catalog/20040.html
David Klassen
> David Klassen wrote:
> >
> > Sea Wasp <sea...@wizvax.net> wrote in message news:<3C6A9B...@wizvax.net>...
> > > You have to decide which laws of physics apply:
> > >
> > > If Conservation of Momentum applies,...
> > > If conservation of energy holds,...
> >
> > I don't see where either of these must be true.
>
> Um, there's a word. "If". You may notice that I use this word above.
>
> This word implies that we are considering possible alternative
> versions of the problem. "If" generally does not mean "this must be
> true", at least not in the version of English that exists in the
> universe I thought I was posting to.
The contrast is that it can be *neither*. Since the system of interest,
the tossed and now moving pebble, is NOT isolated from the outside world,
then there is no reason to assume either of these conservation laws holds.
We have an outside-the-system interaction that changes the system. Momentum
conservation requires that there be no outside-the-system forces acting
on the system and energy conservation requires that there be no "magically"
added mass/energy from outside the system of interest. But this is exactly
what we have: mass magically added to the system of interest.
> Since we do not know which version of physics, if any, the original
> person was using in his world, we must consider ones in which CoM and
> CoE hold, versus ones in which they do not.
CoM or CoE are not true if there is an external-to-the-system force
or energy/mass interaction. CoM would hold true if you imagine the
the spell creating a mass of zero (relative) velocity with which the
original mass "collides"; then just do the vector math. CoE *might* hold
true if you assume the spell can not create/destroy mass energy in the
universe and thus the mass added must be converted from the KE of the
object. However, we have apriori "proof" that magic *can* create mass/energy
from "nothing", so CoE need not hold true.
> > Constant velocity isn't a law here. It is merely the consequence of the
> > kinematics of the problem. Position as a function of time is not dependant
> > upon mass so long as there are no in-system interactions.
>
> Er, seems to me you'd have to have ONE of the above choices, or else
> say "it does whatever amuses the GM the most at the moment" and have
Nope. Just say that the new mass is created by the energy of the spell
itself. Mass just magically increases along with momentum and KE. Thus,
the trajectory remains unchanged.
Sea Wasp
Actually, we do NOT have that proof.
Nor do I have a statement from the original person that this is, in
fact, how magic works in his universe. Therefore, I cannot assume it.
F'rinstance, conservation of mass is certainly used in MY universe.
A spell that SEEMS to create more mass is taking it from somewhere.
It may be coming from another of the infinite dimensions -- and in
fact probably is -- but it's not actually CREATING it. Viewed locally
and with blinders on, yes, but it isn't negating the conservation of
momentum; as a default, the new mass brought in has zero momentum with
respect to the caster.
Locally, it may seem to violate the laws, but globally it doesn't.
>
> > > Constant velocity isn't a law here. It is merely the consequence of the
> > > kinematics of the problem. Position as a function of time is not dependant
> > > upon mass so long as there are no in-system interactions.
> >
> > Er, seems to me you'd have to have ONE of the above choices, or else
> > say "it does whatever amuses the GM the most at the moment" and have
>
> Nope. Just say that the new mass is created by the energy of the spell
> itself. Mass just magically increases along with momentum and KE. Thus,
> the trajectory remains unchanged.
No thank you. If I allow spell energy to convert to mass so easily, I
would have a hell of a time explaining why you can't nuke cities at
fifth level. A few pounds of mass here, a few pounds of mass there...
I ain't touching THAT one. Converting matter to energy is NOT a
trivial event, nor is the reverse. My mages are NOT wielding (at least
at normal levels) sufficient power to level continents.
Martin Leslie Leuschen
: My mages are NOT wielding (at least
: at normal levels) sufficient power to level continents.
Don't have the guts to run a truely high-power game, eh?
<g,d,r>
martinl
Sea Wasp
MUNCHKIN!!!
>
> <g,d,r>
Yeah, that's right, you run!