
An astronaut plays with water in micro-gravity - ColinWright
http://io9.com/5930340/watch-an-astronaut-turn-himself-into-a-floating-head-in-space
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vmilner
Great :-)

<pet_peeve> Can we say "free-fall" though? The ISS has at least 90% of the
gravity we have down here... not very micro! </pet_peeve>

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ColinWright
Well, yes and no. When you are in free-fall, and everything around you is in
free-fall, then in that frame of reference there is (to first order) no
gravity. The slight variations caused by different altitudes and so forth lead
to tidal effects, which are really small, and hence we (using the editorial
"we") refer to them as "micro-gravity".

Of course, in truth there is nowhere in the universe that has absolutely zero
gravity, so the terms "X-gravity" for X chosen from { "zero", "micro" } become
useless unless we agree to use them somewhere, and the free-fall environment
seems pretty much as good a place as any to use these terms.

But this actually amounts to an admission that you are right, but that I'll
probably still use the term "micro-gravity".

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vmilner
Fair enough. I guess I always think of "gravity" as "gravitational field
strength caused by matter" (the gradient of "Einstein's rubber sheet", if you
like), so a free-falling object near Earth will always have plenty.

~~~
ColinWright
I think of it like that, but then relativize it to the motion of the reference
frame.

To see how, take the usual visualization of the gravity well and mark on it
where you are. Then take a tangent plane at that point. The gravity well is
below that plane, and hence falls away from you, along the line towards and
away from the gravity source, but is above the plane in the fore/aft
direction. This explains why tidal forces act inwards in a plane perpendicular
to a line joining you with the gravity source, but outwards along that line.

It's a useful visualization.

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kine
This is unreal. Great find!

