B What does a helium balloon do on the ISS vs in open space?

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What does a helium balloon do on the ISS . most say, its a 0 g environment, but is it really the same as intercosmos travel "0g"?
I saw a video of helium ballons on the vomit comet, where during the "0 g phase" the helium balloons went to the floor, while all other things floated. I would think for the same reasons, on the ISS space station, the artificial "0g" environment due to the centripetal force from gravity, canceling out the earths force on the ISS due to gravity, that the helium balloons would hit the floor as well.
I would then think in open space, all balloons would float with near equal characteristics.
true?
 

sophiecentaur

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In the ISS, they have 'microgravity' rather than none at all. The floor is going 'slower' than if it were in its own orbit and the ceiling is going 'faster' than it would naturally. So there is actually a gradient of apparent gravity for things that are fixed and floating things will actually drift about (slightly) as they would naturally follow the orbit at which they were let go of.
It's something the crew seem to get accustomed to pretty easily, I believe. It is not good to go to sleep unless you are tethered in a sleeping bag.
 

DaveC426913

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I saw a video of helium ballons on the vomit comet, where during the "0 g phase" the helium balloons went to the floor, while all other things floated.due to gravity, that the helium balloons would hit the floor as well.
It is much easier to think of what a helium balloon will do if you remember that all the air around it is trying to do the same thing. If the vomit comet goes into negative g, then the mass of air will rise, forcing the lighter balloon down. Same thing with a balloon in an accelerating car.

In the ISS, first think about what the mass of air will do, then figure that the balloon will do the opposite.
 

sophiecentaur

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If the vomit comet goes into negative g,
The situation in the Vomit Comet is a bit difficult to analyse because the 'zero g' situation doesn't last long and there will be some transitory air currents to upset the situation. ISS is a better model to use, I think; with a 90 minute orbit time and a circular orbit there would be time to observe some more subtle effects.
But, of course, the bottom line is that everything in theory will be subjected to zero g so there would be no 'weight forces') A 10kg mass in one hand and the balloon in the other . . . . weird.
 

A.T.

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...during the "0 g phase" the helium balloons went to the floor...
Then it wasn't exactly 0g, or the balloons had an initial velocity from before the 0g phase.

...due to the centripetal force from gravity, canceling out the earths force on the ISS due to gravity...
That makes zero sense.
 

Andy Resnick

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I saw a video of helium ballons on the vomit comet, where during the "0 g phase" the helium balloons went to the floor, while all other things floated.
I've been on the vomit comet many times and this doesn't make sense since bouyancy forces are absent in free-fall. That said, the transition to free-fall is neither instantaneous nor perfect (in spite of the excellent pilots). Can you please post a link to this video?
 

anorlunda

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It is much easier to think of what a helium balloon will do if you remember that all the air around it is trying to do the same thing.
I think that is the real answer. The balloon is just a visual marker telling us what the air is doing. It is illustrated in this video.

 

Ibix

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My expectation would be that the g force is zero in both the ISS and the vomit comet (@Andy Resnick's caveats aside), so there would be only tidal forces. These would make air slightly (very, very, slightly) denser at the highest and lowest altitude parts of the station/plane and minimum at the middle. Consequently I'd expect Archimedes' Principle to lead to the balloon floating around half way "up" the cabin.

The tidal forces are tiny for the Earth, so I'd actually expect the balloon to be moving hither and yon as air currents take it. And in the not-quite-free-falling vomit comet, I'd expect it to move opposite to dense objects, as in @anorlunda's video.
 

DaveC426913

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The balloon is just a visual marker telling us what the air is doing.
I like that even better.
It puts the focus on the air; the balloon only reacts.

(As evidenced by the fact that, if you removed the air, the balloon would plummet to the ground.)
 

DaveC426913

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I've been on the vomit comet many times
I've always wondered about this zero-G thing, but never had anyone to ask till now:

Isn't the effect indistinguishable from simply plummeting in freefall? How do you get from 'the plane has dropped out from under me and I'm going to plummet to the ground' to 'I am comfortably floating'?

I mean, it's the same hair-raising effect as a plane take-off when it throttles back at altitude, and you feel like you're going to slide backward into the ground, right?

Does it take a while to make that mental leap? Does it stay once you've got it? or do you have to get used to it every time?
 

A.T.

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I mean, it's the same hair-raising effect as a plane take-off when it throttles back at altitude, and you feel like you're going to slide backward into the ground, right?
When you push off a trampoline, you are also in free fall for a few moments. It's not that "hair-raising".
 

sophiecentaur

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When you push off a trampoline, you are also in free fall for a few moments. It's not that "hair-raising".
Did you ever drop off a ten metre diving board? Hair raising or what?
 

A.T.

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Did you ever drop off a ten metre diving board? Hair raising or what?
The transition from 1g to 0g is the same regardless of the jump height. So the fact that a 10m jump is more scary than a 1m jump is not because the change in g-forces.
 
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I've always wondered about this zero-G thing, but never had anyone to ask till now:

Isn't the effect indistinguishable from simply plummeting in freefall? How do you get from 'the plane has dropped out from under me and I'm going to plummet to the ground' to 'I am comfortably floating'?

I mean, it's the same hair-raising effect as a plane take-off when it throttles back at altitude, and you feel like you're going to slide backward into the ground, right?

Does it take a while to make that mental leap? Does it stay once you've got it? or do you have to get used to it every time?
as was quoted, the 'hair rasing effect" is due to free fall. you get launched from a big tieter tauter , as soon as you are ejected, you are decelerating at 1g and at the apex height, you start to fall downward with a 1g acceleration. (correct?). the only reason your hair falls or floats is due to air drag.

my question is during a freefall, why dont you feel the acceleration, after all you have a force due to gravity acting on you, which means you are accelerating, but you feel like you are floating. of course as you fall from an airplane, you accelerate, ONLY to the point you reach terminal velocity.. (where you are not accelerating).. at this point do you now have weight? like on an elevator that has reached top downward speed, it is not accelerating anymore so you have weight based on 1g with respect to the floor of the elevator.
 
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The transition from 1g to 0g is the same regardless of the jump height. So the fact that a 10m jump is more scary than a 1m jump is not because the g-forces.
but jumping from an airplane, what happens at terminal velocity, when the force due to gravity is matched by the drag force and you are no longer accelerating... do you feel your weight again?.... doesn't the vomit comet, in order to create zero g have curved parabolic flight path which basically is like a vertical launching of you in a container, which has no acceleration force after the launch, so you experience free fall on the way up and on the way down. in this example, the horizontal component is missing (the airplane horizontal velocity) . am i thinking of this correctly?
 
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DaveC426913

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Sorry, by 'hair-raising effect' I meant 'poop my pants for fear of falling to my death' - not literally raising my hair.
 

DaveC426913

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When you push off a trampoline, you are also in free fall for a few moments. It's not that "hair-raising".
Sure, but that isn't really the same sensation as being in an elevator that suddenly plummets. There's panic there. Partly because it lasts significantly longer than jumping off a trampoline. And you're aware you're picking up speed that whole time.
 

DaveC426913

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but jumping from an airplane, what happens at terminal velocity, when the force due to gravity is matched by the drag force and you are no longer accelerating... do you feel your weight again?
Well, you'd have a tough time feeling weight while spread-eagled and moving in a 180mph wind. How would you test that you have weight?

But you can do it. Imagine you are plummeting feet-first, and you've deployed a skateboard ahead of you. The skateboard, when aligned properly, will be braked by the wind. You will (in theory) be able to stand on it, and keep it pressed to the bottom of your feet. If you replaced the skateboard with bathroom weigh scale, it would display a measurable weight.
 

Ibix

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my question is during a freefall, why dont you feel the acceleration, after all you have a force due to gravity acting on you, which means you are accelerating, but you feel like you are floating.
Einstein answered this question by saying that you aren't accelerating in any meaningful sense when you are in free-fall. Instead you are moving in the nearest thing one can have to a straight line in curved spacetime. People standing on the floor are the ones accelerating at 1g under a force from the floor (which, in curved spacetime, turns out not to necessarily entail moving anywhere).
 

sophiecentaur

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Well, you'd have a tough time feeling weight while spread-eagled and moving in a 180mph wind. How would you test that you have weight?
It's like laying on an airbed. You feel pressure all over your lower surface.
 
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Sure, but that isn't really the same sensation as being in an elevator that suddenly plummets. There's panic there. Partly because it lasts significantly longer than jumping off a trampoline. And you're aware you're picking up speed that whole time.
yes, that's the reason and I agree.. its because of the things you see around you , but its still a rate of change of velocity that stays constant, correct?
 
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Then it wasn't exactly 0g, or the balloons had an initial velocity from before the 0g phase.


That makes zero sense.
sorry, I misspoke. I was thinking and wanted to say that the ISS has centrifugal force equaling the centripetal force, canceling them out so there is weightlessness. ……..

as far as the balloons sinking (helium on the vomit comet), I think what could have happened is that the air in the cabin had a pressure gradient... agreed... then, as the plane does the parabola, the gradient disperses…. meaning, during the initiation of 0g, the higher density floor air goes up toward the ceiling to equalize and through this initial movement , the helium balloons react to this air movement in a negative way, similar to the balloon going forward in a car when decelerating. I guess the test would be when it stabilized, the helium balloons should then continue to move in the direction they are moving until slowed by air resistance or hitting an object in the plane.
 

russ_watters

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I think that is the real answer. The balloon is just a visual marker telling us what the air is doing. It is illustrated in this video.

I like that even better.
It puts the focus on the air; the balloon only reacts.

(As evidenced by the fact that, if you removed the air, the balloon would plummet to the ground.)
If the air in the car is uniform temperature, it shouldn't move as the car accelerates. The balloon should move back and forth due to it's own buoyancy in the air. E.G., as the air accelerates forward, the gravity vector angles backwards, so the balloon leans forward. It does the opposite of a pendulum hanging from the ceiling.
 

A.T.

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Sure, but that isn't really the same sensation as being in an elevator that suddenly plummets. There's panic there. Partly because it lasts significantly longer than jumping off a trampoline. And you're aware you're picking up speed that whole time.
And on the vomit comet you are aware that you are on a plane, not on an elevator that will crush into the basement.
 
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If the air in the car is uniform temperature, it shouldn't move as the car accelerates. The balloon should move back and forth due to it's own buoyancy in the air. E.G., as the air accelerates forward, the gravity vector angles backwards, so the balloon leans forward. It does the opposite of a pendulum hanging from the ceiling.
That seems contrary to what ive read. the air in the car is at a constant speed or at rest, so with the velocity change of the car showing acceleration, the air mass in the car would compress toward the rear making the air more dense. this makes the helium balloon move toward the front of the car . this is why the gravity vector angling rearward, causes the balloon to move forward.

2 paris of forces.. opposing pressures and opposing forces. the balloon creates a pressure on the air and the air creates a pressure on the balloon and the vector angle of gravity pulls the balloon rear ward with a force that is less than the air mass being pulled rearward, which is its buoyancy force. the force on the balloon (its weight) is less than the force on the mass/volume of air .

its a misnomer to think that helium balloons work "opposite" to gravity, because in a vacuum, a helium balloon will fall like a rock! ;) buoyant force is just the volume and the density of the displaced fluid/gas x gravity. do I have this right??? If so, the buoyant force is the force of how much of a fluid has been displaced. whether it floats, sinks or rises, depends on that weight (force due to gravity on the object)
 

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