Why is microgravity experienced on the ISS despite the presence of gravity?

[jbriggs444]

On Earth you need to maintain that arm at an acceleration of 9.8 meters/sec² relative to free fall. In the space station you have to maintain that arm at an acceleration of 0 meters/sec² relative to free fall.

 

[A.T.]

But why should it be easier to lift the arm on the ISS?

You need only the force for acceleration, not the one for support.

Same goes for moving heavy objects around just with a finger...

Same as above compared to carrying the object on the surface. No friction compared to pushing the object across the surface.

 

[fog37]

How does the feeling of apparent weightlessness (experienced in free fall) and the feeling of being buoyant in a fluid compare to each other? Do the two feel exactly the same?

A scale would tells us that our apparent weight is less when we are bathing in water. The buoyancy surely decreases our contact with a surface. But I wonder if that effect of reduced compression is also transmitted to the internal organs reducing their internal compression.

I know astronauts train in buoyancy environments to simulate weightlessness on earth...

In the case of skydiving, a skydiver jumps out of a plane and free falls feeling weightless for a few seconds. As the upward air resistance force develops and grows the skydiver starts feeling some sense of its weight because the air resistance acts as a supportive force.
Once air drag equalizes the skydiver weight, the skydiver should feel his weight completely as if he was belly down on a soft pillow. The wind provides the overwhelming experience.

The moral of the story is that as soon as any type of supportive force (even smaller in magnitude than our weight) develops and starts counteracting our true weight (our gravitational attraction to earth) we start gaining some sensation of our true weight because of the compressive state we start feeling.

When we are buoyant on top of the water, for example, we should still feel our weight since the buoyant force is a supportive force that matches our weight. It should be the same as when we are laying down on the floor...

Why then is being buoyant compared to weightlessness?

 

[ZapperZ]

Why then is being buoyant compared to weightlessness?

It doesn't compare!

Astronauts train in water to simulate the fact that they will be floating, and that their feet will not be anchored to the ground. It is not to simulate weightlessness, because if this is true, then training in the vomit comet would not have been necessary.

Based on all your questions now, this is why I disagree with Russ and why from the very beginning, I claim that you do not have an understanding of the idea of weightlessness. That should have been addressed FIRST, rather than all the extraneous issues surrounding experiments on the ISS.

Zz.

 

[fog37]

Thanks Zz.

I am open to understanding more. I still not sure which part I think I am grasping yet. Let me try to explain what weightlessness is for me:

it is the apparent feeling being without weight. This happens when we are in pure free fall because our body and its internal organs are all falling at the same rate (acceleration) and they don't "push" on each other, compress each other.

We can only experience our own weight when a supportive force enters the game. The supportive force (for example the normal force of the floor) counteracts our weight and makes us aware of it by feeling our internal organs in a mutual state of stress...

We can only become aware of our true weight (gravitational attractive force) indirectly if there is a force that counteracts it.

what do you think?

 

[mfb]

We can only become aware of our true weight (gravitational attractive force) indirectly if there is a force that counteracts it.

We can be aware of this force then, but not of gravity itself (neglecting tidal acceleration - which would bring the topic back to the microgravity).

Mainly I think there is still atmosphere and therefore friction.

That effect needs months to become notable. It is tiny compared to tidal gravity.

 

[fog37]

Zz,

I found this on Wikipedia:

"...One downside of using neutral buoyancy to simulate microgravity is the significant amount of drag presented by water.[6]Generally, drag effects are minimized by doing tasks slowly in the water. Another downside of neutral buoyancy simulation is that astronauts are not weightless within their suits, thus, precise suit sizing is critical..."

It sound that neutral buoyancy feels very similar to being weightless...What does it mean that the astronauts are not weightless within their suits?

 

[mfb]

If the suit is too large, body parts can be moved up and down in it (in their air inside them) - you still feel the weight of them.
You also don't have a realistic effect on your blood pressure, the the constant feeling of falling down is missing, and other things just because internally gravity acts on your body as normally.

 

[rcgldr]

I'm wondering about the relative magnitude of the small effect of aerodynamic drag due to the thin amount of outer atmosphere that the ISS orbits in versus the small effect of location within the ISS (closest to Earth side, farthest from Earth side).

 

[mfb]

Drag varies a lot with time. To make it worse, it does not actually reduce the speed of the station, it increases it. Starting from a perfect circular orbit, if you add air drag, the station will spiral downwards, getting faster all the time. This speed increase is due to gravity, however, so you don't feel this effect in the station.

Anyway, here is an average: Let's start with the Height as function of time. During October/November, we had a longer period without reboost, the ISS dropped from 404 to 401 km in 1.3 months, or 40 days. Let's assume the orbit is perfectly circular, the eccentricity does not change the result notably.
Specific orbital energy is GMm/(2r), the difference between the two is 0?? Thanks WolframAlpha. The difference is 13 kJ/kg. At a speed of 7.67 km/s, the ISS traveled 2.65*10¹⁰ meters. Diving specific energy loss by length gives an acceleration of 4.9*10^-7 m/s² due to aerodynamic drag.
Note: this is an average value. While in the shadow of Earth (about half of the time of the 90 minute orbit), the ISS rotates its solar panels to reduce drag. During sunlight, it rotates the solar panels to face the sun.Self-gravity is not completely negligible at those levels. 100 tons at a distance of 20 meters produce 1.5*10^-7 m/s² acceleration from the ISS mass. (Note: those values are arbitrary, I don't have a detailed mass simulation available).What about tidal gravity? To get this, we first have to check the orientation of the ISS in space: the image there is in flight direction, and the ISS rotates once per orbit so this orientation stays the same. Much larger image with a different viewing angle.

As you can see, the main structure of pressurized modules is along the orbital track of the ISS. The center of mass is roughly in this part as well, close to a US lab module. Along this path, objects will just follow each other at a constant separation. No tidal gravity.

What happens if we go up by 1 meter? Gravitational acceleration reduces, centrifugal acceleration (in the rotating frame) increases. The difference for 1 meter is 3.9*10⁶ m/s² per meter of height.

What happens if we go one meter to the side? We get a sidewards component of 1.3*10⁶ m/s² per meter sidewards.

As the station is much larger than a meter, tidal gravity wins by a good margin.

On the other hand... as you can see in the height plots, the ISS gets frequent re-boosts to keep its orbit. Those are done with accelerations of roughly 0.02 m/s². If you call them "caused by drag", then drag is responsible for the largest accelerations by far. And certainly to much more fun than tidal gravity.

 

[ogg]

Drag speeds up the ISS? Wouldn't the same logic would require Newton's apple to acquire a tangential velocity as it falls from the tree? It would be great if you (mfb) could point us to a source which derives this. IMHO, microgravity is the better term. I also think talking about "weight" is a bit pointless unless we are assuming a constant g. (or perhaps comparing g and g' between the surfaces of two planets). One old old sci-fi story had the protagonist solve the question of whether he was in orbit or weightless by placing a bunch of ball bearings onto the ceiling (or was it the walls? hmm). If you understand why that would (after perhaps days or weeks) answer the question, you understand the topic. The ONLY things which experience the exact same force (hence have no force differential) when in orbit are on the exact same circle (in the 1 dimensional meaning of the term circle). Above, or below that (assumes a spherical orbit around a spherical grav. potential) the force will be a tiny bit different, to the left or to the right, the paths (great circle) will not be parallel (they'll intersect). Also, the thing that you DON'T have in microgravity is (surface2surface) friction. Moving a large block floating on water (slowly) is nearly as easy as in space, but you still have to accelerate it (and stop it when you get to where you're going).

 

[Redbelly98]

I agree with what you are saying:

weightlessness is a little of a misnomer since the force of gravity is there providing the centripetal force for the ISS to move into its orbit. What there is absence of are the effect of gravity: regardless of the presence of gravity we are able to simulate an environment where the effects of gravity are not present (only in small part)

Well, I disagree with what you're saying. "Weightlessness" is the more accurate term than "zero g", because g isn't zero in this case. Weightlessness refers to the fact that there is no "normal reaction force" that we teach students in intro physics when they have to draw a free-body diagram. So the object "sense no weight", and thus, weightlessness.

But if you do a proper treatment of this right out of intro physics, "zero g" environment and "weightlessness" is no different, the same way you can't tell if you're moving with constant velocity or stationary. So that is why I do not understand the problem here.

Zz.

Weightlessness is the lack of contact forces that support you against gravity. Gravity itself is acting approx. uniformly on your body, so it doesn't cause any "sensation".

Some physics teachers I know refer to freefall and orbital motion as "normalforcelessness" rather than weightlessness. While they say this tongue-in-cheek, it is an instructive term to help think about what is really going on.

These same teachers don't agree on what is meant by "weight". If I recall our conversation correctly, one, who grew up in Russia, uses "weight" to mean what a scale would measure -- what others call "apparent weight". And those others use "weight" to mean "the force due to gravity". The point being, the definition of weight determines whether we can refer to objects in orbit as being weightless -- it becomes a matter of semantics.

 

[mfb]

Drag speeds up the ISS? Wouldn't the same logic would require Newton's apple to acquire a tangential velocity as it falls from the tree?

Coriolis force is doing that, but that has nothing to do with drag.

It would be great if you (mfb) could point us to a source which derives this.

Literally every textbook about orbital mechanics. Lower-energy circular orbits are faster. Drag let's the ISS lose potential energy and gain half this loss as kinetic energy.

Also, the thing that you DON'T have in microgravity is (surface2surface) friction.

You have it if you apply a normal force.
Even worse, you can have cold welding in vacuum conditions.