# Force field physics questions!

Flannel
Hello everyone!

Events in my current webnovel have reached the limit of confidence in my physics reasoning, so I'm here to ask for confirmation of my estimates of what would happen from experimentation with force fields. While the setting is fantasy/magic based rather than superscience, I still want to maintain as much realism as I can.

Tools currently in use:

WolframAlpha Ideal Gas Law
Aqua-Calc volumes to weight conversions

Current questions:

1) Assuming a force field of
• negligible material mass
• with dimensions of 1m x 4m x 9m, for 36 cubic meters volume (yes, a metric scaled 2001/Space Odyssesy Monolith)
• filled with room temperature air at 1 atmosphere, thereby
• massing/weighing 46.54 kg -

how would this object react to an unsteady surface - specifically, resting on the deck of a docked sea ship?

How easily would it tip over? Since it contains an amount of air equal to what it displaces, I have the amateur impression that it would be very unstable.

What would happen as it fell over? My amateur impression, again, is that some of its radial downward momentum would be converted into forward gliding as it closely approached the deck, due to compression of the air beneath against said deck, while dragging its back bottom edge along that surface. Am I correct?

2) When calculating ideal gas law changes, does a decrease in the size of the volume not alter the original pressure constant within the container? That's what I think has been tripping me up when I try to calculate the temperature increase of the air inside when the main character shrinks one of these force fields.

3) More generally, how would a decrease in volume to 20% of the original force field affect the air inside it?

Any help will be gratefully appreciated!

Homework Helper
Hello @Flannel,

How easily would it tip over?
Tip over into what ?

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Gold Member
1) As long as the force-field itself doesn't mass anything, and the internal density is the same as external, it's not going to "rest on" anything, nor "fall down". Think a toy balloon with just enough Hydrogen in it that it floats. You've still got some 100lb of inertia, though.

Depending on the properties you give the field, you might get it to stick to a surface.

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Lren Zvsm
Flannel
Tip over into what ?

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Thank you for the welcome!

Tip over onto the deck below.

Flannel
1) As long as the force-field itself doesn't mass anything, and the internal density is the same as external, it's not going to "rest on" anything, nor "fall down". Think a rubber balloon with just enough Hydrogen in it that it floats. You've still got some 100lb of inertia, though.

Depending on the properties you give the field, you might get it to stick to a surface.

So for it to settle, I could make the mass of the barrier "material" itself non-negligible instead, right?

I can work with that. In fact it will probably even make things easier on me going forward, since I'll have to set constants for barrier durability relative to thickness just to begin with. Then I can use those for mana expenditure calculations.

Baseline max resistance of five atmospheres would be 73.5 psi, which is. . .between half and 2/3rd average human punching force. Yes, good! Not too strong, but that should scale well.

Flannel
Aaaah! Where did the other reply about volume/temperature go? Noooo! That one was super helpful too! I didn't imagine it, right?

I still have to figure out all sorts of things about heat/cold/pressure/vaccuum damage potential!

Homework Helper
Tip over onto the deck below.
I am afraid I don't understand. Wouldn't the air on the deck below be in the way somewhat ? Or does this all take place in a vacuum ?

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Gold Member
Aaaah! Where did the other reply about volume/temperature go? Noooo! That one was super helpful too! I didn't imagine it, right?

I still have to figure out all sorts of things about heat/cold/pressure/vaccuum damage potential!

Here are three ways to look at the problem. Let V=V0/a. I have implicitly assumed that T0 is equal to the external-to-the-field air temperature. A “fast“ compression will look adiabatic.
1) Isothermal compression (air temperature does not change): In this case the relevant equation is PV=kT=constant
PV/P0V0=1
P=P0V0/V
P=aP0
2) Adiabatic compression (there is no heat flow). In this case the pressure is determined by
PVϒ=constant=P0V0ϒ
P=P0(V0/V)ϒ=P0aϒ
for air ϒ=1.4
Now use PV=kT
PV/P0V0=T/T0
T=T0aϒ-1 (T is measured in Kelvin)
for air initially at room temperature (298K) and a=5 (volume compressed 80%)
gives T=567K= 561°F
3) You could control the amount of heat flow to let T vary from T0 to the adiabatic temperature. In this case, use
PV=kT
PV/P0V0=T/T0
P=aP0T/T0

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Flannel
I am afraid I don't understand. Wouldn't the air on the deck below be in the way somewhat ? Or does this all take place in a vacuum ?

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Yes, the air below it would. In case you're asking about its expansion from a central point, the generation of the field's 6 connected surfaces is effectively instantaneous in all axes. So the field was generated already resting on the ship's deck - er, stage, that is - below.

And yes, it's happening in a big room. The main character's doing his testing on the stage of the Theater on the maindeck, in the hull's largest area, i.e. under the weather or lowest exposed surface deck.

I'd forgotten about the effective air-buoyancy issue, which is rather embarrassing considering I just had a supporting character figure out lighter-than-air flight from a mere few hints several chapters earlier.