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International Low Gravity Day

Stick some asteroids and comets together to make more moons
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International Low Gravity Day occurs at the same time every year due to the carefully constructed orbits of some new moons made up of old space trash, comets and asteroids.

So once a year these moons on very eccentric orbits dip toward earth and reduce gravity by 50%. Four moons are used to ensure the same experience globally and to not throw the earth off course (we can use them later to move the earth out a bit).

So for a few hours we can run and jump like John Carter on Mars.

Might be a laugh if we all lived underground otherwise; to spare cities of the mile high tidal waves.

bigsleep, Jan 24 2013

Gravity inside a shell http://hyperphysics...nics/sphshell2.html
Scroll slowly to the end. [bigsleep, Jan 24 2013]

[link]






       //mile high tidal waves// - but on the other hand, just think how much fun trampolining would be on this day!
hippo, Jan 24 2013
  

       The N-Prize (c.f) will need an additional rule to make clear that N-Prize attempts on ILG day are not valid.
hippo, Jan 24 2013
  

       Trampolining would not be extra fun lower gravity would let me jump higher but lower gravity would prevent me to jump higher
zeno, Jan 24 2013
  

       You would be able to jump higher on a trampoline, and I think it should be by the same ratio you could jump higher just using your legs. The reason is that all a trampoline does is return the energy of previous jumps to the user. Since the energy returned to the second jump from the first jump would be the same (leg strength - friction losses) you would still (essentially) go twice as high as your legs would allow (at the lower gravity) on the second jump, and three times on the third, etc. (to the point of friction losses exceeding leg energy). Now making sure you hit the trampoline on the way down, that might be difficult.
MechE, Jan 24 2013
  

       As far as the idea. Build a complete shell of dense material around the earth. Now calculate the gravitational affects of said shell on the gravitational experience of someone living on the earth. I'll wait, it's only simple calculus...   

       ...   

       ...   

       ...   

       If your answer wasn't zero, go back and try again.   

       Now do the math for four point sources equidistant in tetrahedral positions above the earth. The math is simpler for any given position, but it will take a while to figure for all of them. Regardless, the result doesn't change. One at a time might do something, but all at once does absolutely no good at all.
MechE, Jan 24 2013
  

       //One at a time might do something//   

       That's what I was thinking, a kind of staggered approach.
bigsleep, Jan 24 2013
  

       The way this would work is through tidal effects. Which are confusing.   

       If you put a dense "moon" close enough to the surface of the earth, the side closer to the moon is getting more gravity from the moon than is the side of the earth that is further from the moon---so objects on the near side are lighter. As the earth rotates around the (center of gravity of Earth and the) moon, the centrifugal slinging forces are stronger on the side further away from the moon.   

       That is a simplification of tidal effects, but it happens. (Around a black hole, BTW, tides will cause "spaghettification".)   

       If you have a balance of moons, the centrifugal aspect will be lost. I agree that the gravity aspect will cancel out, as well.   

       One moon on a low pass would be best. I do not think that tidal waves would have time to really build up (most ocean-tidal activity is oscillatory), but you'll want to plan the route of the low pass for land areas anyhow.
baconbrain, Jan 24 2013
  

       Bust up those tectonic plates! Pour the lava from the fumarole! Give the ground the shakes and quakes! Break the dams, let rivers roll!   

       (attempts to bend one of Bilbo's forks)
lurch, Jan 24 2013
  

       I think trampolines would have to be adjusted to account for the .... no wait, they'd be fine. Your reflexes'd be a bit messed though.   

       The Earth on the other hand, not so much: on a near pass all the rock underneath, solid and liquid, going down to the core weighs 5/6 its usual. Ditto water, suddenly weighing 5/6 what the water surrounding it weighs (yeah I know it's a gradient, still...)
FlyingToaster, Jan 24 2013
  

       //If your answer wasn't zero, go back and try again//   

       I'm having trouble understanding this, which isn't surprising as I don't calculus, but...
If the shell surrounding the Earth had the same mass as the Earth then the only thing keeping Earth-bound objects Earth-bound is the gravitational inverse square law right?
So if the surrounding shell were proportionally more massive that the Earth then things would indeed weigh less... wouldn't they?
  

       ^ Yeah, I'd heard that: doesn't make intuitive sense but... if you were inside a room inside the Earth, 100mi from the core, you wouldn't expect to be pulled up to the ceiling, it'd be the same as being in a room on a 100mi wide asteroid in the middle of nothing.
FlyingToaster, Jan 24 2013
  

       Would there not be some point where outer spherical mass would exceed the inner mass by enough to attract objects upwards?   

       [2fries] Never. See [link] which is intuitively what I thought.   

       It doesn't say it in the idea, but the moons would not be present at the same time. Maybe they pass each side of the planet over the course of the event to not alter Earths orbit.
bigsleep, Jan 24 2013
  

       Since the net gravitational force inside a shell is zero, and the net gravitational force outside a shell is determined by the mass of the shell, I wonder what the experience of traveling from inside to outside (or vice versa) would be like.
ytk, Jan 25 2013
  

       Well I just can't wrap my head around that.
and apparently even if I could, it would exert no force.
  

       what's really annoying is that if you were inside an Earth-sized void inside an Earth-massed shell you'd float to the centre and stay there.
FlyingToaster, Jan 25 2013
  

       [bellauk65] That reminds me of anno I posted which pondered the critical mass of kittens in a ball in space before they turn into a black hole.
bigsleep, Jan 25 2013
  

       //383,555,613,815 kg of mass, which I cannot work out what gravity it would generate//   

       Setting aside grammatical errors for the moment, we have a sphere with a mass of 3.8E11kg. Assuming its density to be about the same as that of water, that equates to a sphere with a radius of about 500m.   

       (Heavens - is that right? All of humanity in a sphere with a radius of 500m? Well, yes, it is.)   

       So, if you were standing on the surface of this sphere, you would experience a gravitational pull of about 0.1N/kg, or about 1% of terrestrial gravity.
MaxwellBuchanan, Jan 25 2013
  

       Is this the moment to point out that one of the useful effects of gravity is to retain your planet's atmosphere?   

       Mars has gravity of about 0.8g and has a thin atmosphere with very little oxygen or water. If you reduce your gravity to 0.5g, quite a lot of your lighter atmospheric gases are going to disappear into space, where the solar wind will push them away. When the gravity comes back, the gases won't.   

       As your species seems quite keen on that "breathing" thing you do, this may prove a trifle inconvenient.
8th of 7, Jan 26 2013
  

       //As your species seems quite keen on that "breathing" thing you do, this may prove a trifle inconvenient//   

       Damnit! You're tipped off Fox News for the first scoop of "An Inconvienient Trifle".
bigsleep, Jan 26 2013
  

       As long as they're allowed only one scoop; hopefully there will be enough left for everyone else at the party.
8th of 7, Jan 26 2013
  
      
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