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Old 01-20-2019, 08:38 PM   #1
CraigR
 
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Default Mass and air density

if a person were teleported from an area, and teleported in his place was an equivalent mass of air (not volume, MASS), how much outward force would be produced? Would it be like an explosion, or just a really strong breeze?
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Old 01-20-2019, 08:52 PM   #2
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Default Re: Mass and air density

Okay, so one quick Google search later: a cubic meter of human flesh is 985kg, and a cubic meter of air is 1.293kg. So, packing a human's mass worth of air in a human's shape would produce a pressure of approximately 761.794 atmospheres, or about 11,427 psi.

So how would that convert to pounds of TNT, or GURPS damage?
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Old 01-20-2019, 09:05 PM   #3
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Default Re: Mass and air density

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Originally Posted by CraigR View Post
if a person were teleported from an area, and teleported in his place was an equivalent mass of air (not volume, MASS), how much outward force would be produced? Would it be like an explosion, or just a really strong breeze?
It'll be a fairly substantial explosion - compressing the same mass of air into your volume requires about 800 atmospheres of pressure. For a typical mass person the energy release is between a 6d x 6 and 6d x 40 explosion depending on your assumptions about how the gas expands - probably toward the low end, because a sudden release expansion will be a lot closer to adiabatic than isothermal. It'll also throw around a significant amount of cryogenic liquid - for an ideal approximation the final adiabatic expansion temperature comes out to something around 50 K, well below the 90 K liquefaction point of air.... Expect anyone within a few hexes to suffer severe frostbite.

Also assuming the air comes from around wherever the unfortunate victim teleported *to*, he emerges in the middle of a 3 hex radius vacuum. The collapse of that is much less explosive, but probably still inflicts a couple dice of damage in free air. If you emerged inside you'll certainly collapse the room, and quite possibly the building, and may well suffer more injury from the flying debris.
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Old 01-21-2019, 08:28 AM   #4
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Default Re: Mass and air density

Awesome! Thank you!
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Old 01-21-2019, 09:12 AM   #5
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Default Re: Mass and air density

Energy is pressure x volume. A pressure of 762 atmospheres is about 76 million joules per cubic meter, and a volume of 56.7 liters (for a 125-pound human) is 0.057 cubic meters. That gives about 4.3 MJ, which is approximately a thousand kcal. I believe that TNT has about 1000 kcal/gram, so that's about a kilogram of TNT, or about 2.2 pounds.
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Old 01-21-2019, 09:38 AM   #6
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Default Re: Mass and air density

Another fairly intuitive way of thinking about why this looks like an explosion is to consider that the first stage of a chemical explosive detonating is converting a solid (which is typically around the density of a person) into the same mass of gas occupying the same volume. The explosion products are also hot, which adds energy, but then we don't know how this process compresses the air into the volume of a person, the air might be hot too.

You'd certainly expect something in the same general range as you'd get from setting off an explosive around the weight of the teleported object.
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Old 01-21-2019, 02:33 PM   #7
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Default Mass and air density

IIRC the density of a human with full lungs approximates 10/11th of water. Hence bodies float (if not high enough in the H20 to avoid drowning.)
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Old 01-21-2019, 02:45 PM   #8
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Default Re: Mass and air density

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IIRC the density of a human with full lungs approximates 10/11th of water. Hence bodies float (if not high enough in the H20 to avoid drowning.)
It depends on the person - fat people float higher, some very lean people sink even with their lungs full of air. When they fully exhale most people have slightly negative buoyancy, so I'd just assume that people average the same density as water, overall.
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Old 01-21-2019, 04:37 PM   #9
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Default Re: Mass and air density

Also remember that most tables will report DRY air. If your swapped volume is humid air there will be more water (and therefore less compression and subsequent reexpansion).
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Old 01-21-2019, 04:44 PM   #10
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Default Re: Mass and air density

Why does the air get compressed at all? If the castling isn't volume-related, but mass-related, then it's not obvious that the swapped-in air needs to show up in the same volume of the departing person. Perhaps it's just distributed over the same volume it came from (much like the human, as it would be disturbing for the human to get compressed during jump just like the air), resulting in slightly higher pressure over a large volume that rapidly dissipates.

It's an imaginary ability, so you can have it work however you like, of course. But it seems odd that the transfer process is so asymmetrical.
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