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There are more than a few scenes in Gravity that I find dubious. Specifically the ones where people are revolving very quickly, either about their own center of gravity or at the end of a rather long pole. I'd guess we'd need to know how long the pole was, but I'm more concerned with a person themselves, just flipping around out there (and I suppose we'd need their height, or does that cancel out?). How fast can they spin until it's night-night? Is it reasonable to assume they should've passed-out in at least one of those scenes?

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I think people can easily endure head-over-heel revolutions in space such as seen in Gravity. I am no mathematician (let alone physicist, both of whom could answer this more accurately) but here is my reasoning (faulty though it may be).

You are assuming that someone would pass out from lack of blood supply to the brain. Given we have absolutely no problems normally at 1 g (the force of gravity on earth), the average non-couch-potato person can tolerate forces of up to 20 G for less than 10 seconds, to 10 G for 1 minute, and 6 G for 10 minutes before passing out.

But g is very dependent on (as you stated) distance from center. It's also very dependent on revolutions per minute.

$$g=\frac{R\left(\frac{\pi\times rpm}{30}\right)^2}{9.81}$$ or $$R=\frac{9.81 g}{\left(\pi\times rpm\over30 \right)^2}$$

g = Decimal fraction of Earth gravity
R = Radius from center of rotation in meters
π = 3.14159
rpm = revolutions per minute

The average North American male is 1.75m tall. That means the distance of the top of the head and bottom of the feet (if stretched out at full length) from center is under a meter (.875m). At 60 rpm's (that's pretty fast), solving for the above, g = 3.52.

g = [.875({3.14159x60}/30)^2]/9.81 = 3.521 (so just over 3.5G)

Gravity at the top of the head and the bottom of the feet would be 3.5G, while the gravity at the center would be zero G. The heart can easily compensate for that for a while. An astronaut, trained for increased Gs, can do well for longer.

Much worse than too little blood to the head is too much, which is what I would be worried about. Eventually, slightly less than half the blood volume would end up below the waist with the feet feeling the most pressure, and the rest above, with the brain being engorged with blood, which can cause hemorrhage (not taking into account here precise fluid physics). I'm not sure how this would play out in space over longer periods of time, but survival would be the norm for the scenes we see in Gravity.

An Air Force physician named John Stapp was the first to determine just what humans could take in the way of G forces. On his final run, firing nine solid-fuel rockets, his "sled" accelerated to 632 miles per hour in five seconds, slamming him into two tons of wind pressure, then came to a stop in just over one second. A witness said it was "absolutely inconceivable anybody could go that fast, then just stop, and survive." But Stapp did—in fact, he went on to live another 45 years, dying quietly at home in 1999 at the age of 89—and he experienced a record-breaking 46.2 G's. For an instant, his 168-pound body had weighed over 7,700 pounds. Stapp's efforts put him on the cover of Time, and he was called "The Fastest Man on Earth." More importantly, his work led to greatly improved safety in both planes and cars, and he gave us a much-improved understanding of human tolerance to G forces.

All About G Forces

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    $\begingroup$ Very interesting that you would have to flip three times a second to put you into the one minute danger zone; not as dubious as I thought. $\endgroup$ – Mazura Feb 13 '15 at 22:56
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    $\begingroup$ Nice fact too :) $\endgroup$ – WYSIWYG Feb 14 '15 at 5:00
  • $\begingroup$ I think "too much blood to the head" is in fact the problem that would be experienced, since centrifugal force would force the blood away from the axis of rotation and thus towards the top and bottom of the body. Per this, the tolerance limit for that would be only ~2G, making 60rpm readily fatal. That tolerance is less time-dependent than most. Of course, the rush of blood to the legs as well may play with that result, as may the high centre-of-mass of a human; I'd also be interested to see the effect of the spinning on venous return, cardiac preload etc. $\endgroup$ – Watercleave Feb 16 '15 at 17:28
  • $\begingroup$ @Watercleave - According to xkcd, Dr. Stapp would not have survived (even though he references Stapp's work. His stick figure model doesn't mention time. $\endgroup$ – anongoodnurse Feb 16 '15 at 21:00
  • $\begingroup$ @Watercleave - I alluded to this in my answer; I also mentioned that more like half the blood would go to the head; that factors in as well. xkcd is incorrect. Refute me with NASA's numbers (which xkcd references but doesn't use) which states that -4 - -6Gz (that's 6G blood to head position) would be uncomfortable but tolerable for 6 seconds. -3Gz doesn't mention blackout at all. Also, it doesn't change my conclusion: that for what we see in Gravity, the spins would be survivable. $\endgroup$ – anongoodnurse Feb 16 '15 at 21:04

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