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zompist bboard • View topic - Movement in Lower Gravity

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PostPosted: Tue Jul 26, 2011 4:03 pm 
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PostPosted: Tue Jul 26, 2011 4:06 pm 
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PostPosted: Tue Jul 26, 2011 4:12 pm 
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PostPosted: Tue Jul 26, 2011 4:19 pm 
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PostPosted: Tue Jul 26, 2011 4:37 pm 
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1. Gracility: being robust has a cost. If you have massively thick legs, for instance, you need extra food and oxygen to maintain all that matter. This is only wise if that extra matter is actually useful - ie if it stops you falling over. In your world, a) there would be less threat of falling, and b) there would be less damage from falling anyway. So there would be less energy expended on not falling: ie creatures would be more gracile. Of course, if you also make it colder, that would work in the other direction. I think what you'd tend to get is animals with very short legs? Because they'd want to keep the mass of the legs minimal to reduce costs, but they'd also want to reduce the ratio of surface area to mass. In this world, bigger legs would be a big drain on resources. [Of course, there are reasons to the contrary, so they won't all be snakes...] Not sure about this, though...

2. Hopping: no, I don't see why it would be much more common compared to, say, galloping. Although cold climate might help, I suppose - though to my knowledge it hasn't done on earth. If you did have hopping, that would counteract the short-legs argument some, I suppose.

3. Flight: humans in such an environment would probably never discover LTA flight, since they'd discover HTA flight so early on in their history. Consider: on earth, it is possible for humans to fly through human muscle-power - but only just (no flights under 1km, iirc), and it wasn't until the 20th century that we could do this. It wouldn't take a much thicker atmosphere or much less atmosphere to make this seriously viable.

4. The bigger point: nobody can answer your question, because when you're talking about a planet SO dramatically unearthlike it's not really possible to sensibly extrapolate.

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PostPosted: Tue Jul 26, 2011 5:40 pm 
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1. Hmm... yeah, ceteris paribus legs should be thinner, at least for organisms that don't depend too much on speed and maneuverability, so we'd get elephants, cows and rhynoes with thinner legs, probably covered in fur. Heavily leg-robust designs, like elephants, would evolve into a more gracile form... say, antelope-like... something like a huge elephant-headed tapir. long legs are still useful, under these circumstances: remember, they help you move faster, so overall leg lenght would probably be similar as on earth, they would just tend to be thinner.

2. Yup, hopping would be as affected as other hoppy means of locomotion, like galloping. Overall we'd see stuff like elephants and rhynos galloping, probably moving more like cows and horses. I wonder if this would have an effect on, say, the comparative advantages of chasing over long distances versus ambushing... jumping over something would probably be a better idea, since falling is way less dangerous.

3. discovering HTA flight? sure, muscle power in these circumstances becomes more than enough to make a man fly, but its the technology of the thing which makes it hard, I think. Gliding, on the other hand, becomes a good idea, like jumping off a cliff or tower with some sort of winged contraption strapped to one's back and safely landing a few kilometers away, but from there to mustering human power into flight would require some means to turn leg movement into something that can provide thrust, like a propeller, cause bird-like flight is *waaaay* too complex for anyone without space-age tech to imitate. How would anyone with iron-age tech make a human-powered flying machine, I wonder... was the thrust-generating effect of a propeller known to, say, greeks?

4. Yeah, I know... its fun to try!

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PostPosted: Tue Jul 26, 2011 6:48 pm 
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This whole thread raises an interesting question.

Is there a general formula relating atmospheric pressure to composition, height, temperature, and gravity?


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PostPosted: Tue Jul 26, 2011 6:55 pm 
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PostPosted: Tue Jul 26, 2011 7:34 pm 
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PostPosted: Tue Jul 26, 2011 8:16 pm 
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PostPosted: Tue Jul 26, 2011 8:24 pm 
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Well Suenu is a cold place: even in the equator, near the coastline, it gets pretty cold, way below zero... say, -10. I'm not sure, but aren't bugs kind of bad at cold?

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PostPosted: Tue Jul 26, 2011 8:31 pm 
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PostPosted: Tue Jul 26, 2011 8:36 pm 
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PostPosted: Tue Jul 26, 2011 8:44 pm 
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PostPosted: Fri Aug 05, 2011 12:56 pm 
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I noticed a string of variables thrown about concerning atmosphere, but it apparently has been forgotten that incident solar winds significantly affect atmosphere. Being far away from a star, as well as being in orbit around a dimmer star helps, but the real kicker is the existence of a magnetic field. Earth and Venus both are protected from Solar wind by self-generated fields. Titan is also protected, at least by Saturn's (I'm not sure if it has its own field as well) field. I've seen it hypothesized that Mars used to have a significant atmosphere, but then lost its magnetic field, and the atmosphere with it.

Having said that, a planet half the size of Earth would have to be very young (imminently possible, since young is several millions of years) so that any molten core it has remains molten, or subject to powerful tidal forces, such as from a close-orbiting binary planet, or by turning the panet into a moon of a gas giant (remember that the moon was unable to maintain its molten core though it is subject to tidal forces from Earth) (also remember that very large gas giants, such as Jupiter, exude massive i.e. lethal radiation).

I'd say stick with young, it makes sense if this system is in a nebula, and it's simpler, too. And now that I think about it, the magnetic field from a gas giant could easily screw up terrestrial biology, depending on their sensitivity to it. Even if it doesn't answer your original question, it adds a piece of history to the place.

Since I'm here, though, I might as well tackle gracility: animals that can use more strength will develop more strength, and the lower grav will allow them to keep their speed (imagine a lioness hunting down elephants); animals that could use more speed will lose muscle mass, but that won't affect strength (you've already imagined elephants filling a gazelle niche). Here's the kicker, though: it's known that humans lose muscle mass in zero-g, this is because the muscles do not get used as heavily. The same may well hold true in a 1/2-g environment, but if you've specifically seeded/engineered megafauna, then the usual atrophy from transplantation might be ignored. Ultimately, if you can make a halfway-reasonable explanation, you can do what you want and the reader will fill in the gaps, or at least assume the gaps can be filled by some science that's over their head.

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PostPosted: Fri Aug 05, 2011 3:55 pm 
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The relationship between scale height and gravity is pretty much inversely proportional at the same temperature (though there are confounding issues since the altitude to temperature to pressure ratios are non-linear).

Basically, at 1 atm, doubling the gravity more or less halves the scale height, and vice-versa.

So a planet with 0.5 Earth gravity but similar surface pressure would have an atmosphere something around twice as deep. Also, since pressure falls off exponentially, a moderate increase in the surface pressure makes the entire atmosphere thicker by that factor, but doesn't increase the depth of the atmosphere by as much (mathematically, the change in depth in units of scale height is proportional to the log of the change in pressure).

I agree that heavier-than-air flight would be easier to discover with a thicker atmosphere/lighter gravity, but also lighter-than-air flight as well. Your lift per volume of lighter-than-air gas is proportional to both pressure and gravity, so if your gravity is 0.5x and your pressure is 2x, you get the same number of pounds of lift. But in 0.5x gravity, your airship only weighs half as much, so it's still a huge win.

Or to put it differently, lighter gravity and thicker air both help heavier-than-air flight. Thicker air helps lighter-than-air flight. But lighter-than-air flight is simpler - you don't have to discover how a wing works in order to do it.


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PostPosted: Mon Aug 08, 2011 1:18 pm 
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PostPosted: Mon Aug 08, 2011 10:23 pm 
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PostPosted: Mon Aug 08, 2011 11:43 pm 
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1, 2- terrestrial animals transplanted not long ago

questions:
1 5C°
2. no seasons. not significant ones at least, very small tilt

aas for CO2, irradiation, and stuff... I dunno, I'm gonna go with authorial fiat here; the planet's just cold, with a smaller, dimmer star. As for the exact figures, how strong the greenhouse effect is, and stuff.... hummm... yeah, no, I don have 'em.

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PostPosted: Tue Aug 09, 2011 7:36 pm 
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PostPosted: Wed Aug 10, 2011 8:31 am 
Lebom
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Isn't carbon dioxide supposed to be dangerous to animals when taken in beyond a certain amount? I know that enough of it will give you hallucinagenic effects and a slight burning sensation in your lungs.


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PostPosted: Wed Aug 10, 2011 10:59 am 
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PostPosted: Wed Aug 10, 2011 1:12 pm 
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Well, more like -78C° if google is correct. Still, earth reaches those temperatures sometimes, but earth has a less dense atmosphere and less greenhouse effect, so temp shouldn't vary as dramatically on Suenu. Also there's no continents at the poles but there's one pangea that is near the south pole at one latitude. I figure that causes two superficial sea currents along the south coast of the pangea which tend to drag ice from the poles into the ocean. Also, since the rotation is so slow, ocean currents work differently: at the surface the dayside pushes warm water towards the nightside through the poles, and part of that current's bound to go through the poles, in turn warming them. not really warm, but probably enough to melt the eventual dry ice patch that forms... and indeed they form, I'm sure.

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PostPosted: Wed Aug 10, 2011 4:04 pm 
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PostPosted: Wed Aug 10, 2011 10:06 pm 
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