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 The Trek BBS Simple gravity question

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 March 9 2010, 05:49 AM #1 The Borgified Corpse Admiral     Location: Ouch! Forgotten already? You were just down there 20 minutes ago. Simple gravity question I'm collaborating with some other writers on a sci-fi webseries. It's set on a heavy gravity prison planet. My question is, how heavy can the gravity get before a normal, Earth-bred human is rendered incapable of functioning normally? One draft mentions that the planet is 7 times the size of Earth. If we're talking 7 times Earth's mass, thus 7 times its gravity, that sounds rediculous to me. Am I wrong? __________________ Kegg: "You're a Trekkie. The capacity to quibble over the minutiae of space opera films is your birthright."
 March 9 2010, 07:56 AM #2 psCargile Ensign   Location: GA Re: Simple gravity question You are right. Seven gees is exceptionally high. A 200 lb person would weigh 1400 lbs. Even at 2 g that's 400 lbs. I would think that if you aren't able to support twice (or more) your weight for a long period of time, you'll be incapacitated rather quick. The heart is going to have a hard time pumping blood. The best analogy is what combat pilots can endure. Do a search for that for better explanations than what I can give. Is there a reason why the prison planet needs to be a high gravity world? If it were just to make it exotic, I'd drop the whole notion. But if you need it, anything over 1 to 2 gees will be suitably uncomfortable until a person acclimates. 3 gees might be pushing it. At 7 to 9 gees, its not so much a prison planet as a means of execution.
 March 9 2010, 08:20 AM #3 The Borgified Corpse Admiral     Location: Ouch! Forgotten already? You were just down there 20 minutes ago. Re: Simple gravity question ^Honestly, I think they're just trying to make it exotic. Certainly, making the gravity slightly heavier would add to the hostile nature of the planet. But I'm thinking 1.25 gees would probably be sufficient. 2 gees is probably as far as I would go. Any further and it would also start interfering with some of the cool martial arts fighting & such. I'm just trying to make it a scant more plausible. The central conceit of the series is such strange fantasy to begin with, we probably don't want to compound it with crap science. __________________ Kegg: "You're a Trekkie. The capacity to quibble over the minutiae of space opera films is your birthright."
 March 9 2010, 08:23 AM #4 The Borgified Corpse Admiral     Location: Ouch! Forgotten already? You were just down there 20 minutes ago. Re: Simple gravity question BTW, the dialogue in the draft merely says the planet is 7 times the "size" of Earth. I suppose that could be referring to something else, like surface area, not mass. What's the likelihood of a planet with an Earth-like mass but 7 times the surface area? __________________ Kegg: "You're a Trekkie. The capacity to quibble over the minutiae of space opera films is your birthright."
 March 9 2010, 03:14 PM #5 Chaos Descending Vice Admiral     Location: Grand Canyon State Re: Simple gravity question Earth-like mass but 7 times the surface area would mean a planet with a much greater radius, and therefore a much lower surface gravity. Also, in regards to original post, 7 times the mass would not equate to 7 times the gravity unless the volume (and therefore the radius) of the planet was exactly the same as Earth. The force of gravity is proportional to the mass but inversely proportional to the square of the radius. __________________ "Romanes eunt domus" - Brian
 March 9 2010, 03:37 PM #6 Deks Rear Admiral Re: Simple gravity question How much of a time frame would be needed for a Human to adapt from a 1 G environment to 3 G for example? If/And once the individual in question adapts ... can you then try to put the 3G adapted human into a 4, or 5 G environment and see if they can adapt with sufficient amount of time? __________________ We are who we choose to be but also have predefined aspects of our personalities we are born with, and make art that defines us.
 March 9 2010, 05:35 PM #7 Chaos Descending Vice Admiral     Location: Grand Canyon State Re: Simple gravity question I don't know that a human being could EVER adjust to a 3g+ environment. A 200 pound man would have to get used to weighing 600 pounds. I just don't think that our knees, back, etc. are designed to withstand that sort of load. __________________ "Romanes eunt domus" - Brian
 March 9 2010, 08:05 PM #8 Ronald Held Rear Admiral   Location: On the USS Sovereign Re: Simple gravity question Why don't you be vague about the planetary physcial parameters and just state what the acceleration is in gees.
 March 9 2010, 08:28 PM #9 sojourner Admiral     Location: I'm at WKRP Re: Simple gravity question Just a note, this thread would be better in science and technology than trektech. With that out of the way. Don't forget about how a high gravity world is going to effect your martial arts. You would see very few throws, jumping manuevers, high kicks, etc. Most of it would be grapples, sweeps, etc. __________________ Baby, you and me were never meant to be, just maybe think of me once in a while...
March 10 2010, 02:11 AM   #10
The Borgified Corpse

Location: Ouch! Forgotten already? You were just down there 20 minutes ago.
Re: Simple gravity question

 Chaos Descending wrote: Earth-like mass but 7 times the surface area would mean a planet with a much greater radius, and therefore a much lower surface gravity. Also, in regards to original post, 7 times the mass would not equate to 7 times the gravity unless the volume (and therefore the radius) of the planet was exactly the same as Earth. The force of gravity is proportional to the mass but inversely proportional to the square of the radius.
OK, so, because I'm not always a math whiz, what would be the approximate surface gravity of a planet of Earth-like density with 7 times Earth's mass & 7 times Earth's volume?
__________________
Kegg: "You're a Trekkie. The capacity to quibble over the minutiae of space opera films is your birthright."

 March 10 2010, 02:24 AM #11 sojourner Admiral     Location: I'm at WKRP Re: Simple gravity question I am probably wrong, but I am guessing 1g. __________________ Baby, you and me were never meant to be, just maybe think of me once in a while...
 March 10 2010, 04:42 AM #12 T'Girl Vice Admiral Re: Simple gravity question If the world had seven times the volume of Earth the diameter would be 32,970 miles, with a total surface area of 3,414,976,873.7 sq. miles, that's 17.34 times the area of Earth. If this world had Earth's mass the gravity at sea level would be 0.5665 gee. If the world had Earth's density the gravity would be 2.87 gees. Last edited by T'Girl; March 10 2010 at 05:15 AM.
 March 10 2010, 07:44 AM #13 SpyOne Commander Re: Simple gravity question The effect of high gravity on people for extended times is largely guesswork: we haven't spent a lot of time having test subjects living in centrifuges to see what happens. IIRC, 1970s scifi had human colonies on worlds up to 2.5g on a regular basis. By the late 80s, scifi games were predicting that above 1.2g would interfere with your ability to sleep. In the 1970s, scientists at the University of California at Davis raised some chickens in centrifuges specifically to see if the predictions were accurate. After living in 2.5g for months, chickens were 3 times stronger, had low body fat, had improved endurance, and their hearts were moving much greater volumes of blood. I also should point out that a world much larger and more massive than the Earth will probably also be denser than the Earth: the gravity and the weight of the rock and stuff that makes up the planet will be squishing the stuff at the center much more than the Earth does. So a planet as described above (seven times the volume of Earth, at least as dense as Earth) would likely be even denser than Earth, and thus have higher gravity.
 March 11 2010, 01:04 AM #14 psCargile Ensign   Location: GA Re: Simple gravity question Borgified Corpse (neat name LOL) I have a calculating spreadsheet at home (at work as I post) that I created for determining planetary attributes based on size and mass. This is something I made for my own fiction and creating solar systems in Celestria. I can email this to you if you want. Does a planet with 7 times the amount of mass as Earth have 7 times the pull of gravity, no matter it's radius? Let's find out: g= G*(m1/r^2) where by G is the gravitational constant 6.6742x10^-11, m1 is the mass of the Earth in kilograms, 5.9736X10^24, and r is the Earth's radius in meters, 6.37101X10^6. For 1g that equals 9.822 meters per second per second acceleration. 7 times the Earth's mass is 4.18152x10^25, which gives, 68.757. Divide that by 9.822 and you get 7. 7 times the Earth mass and 7 times the Earth radius gives 1.403 m pers s squared, or .143 g. This would be a planet a lot less dense than Earth. 7 times the mass of the Earth and 2.4 times the radius would give you 1.21 g. http://en.wikipedia.org/wiki/Earth%27s_gravity for the formula. Plug that into a spreadsheet and play.

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