Zero Gravity or microgravity?

1. Feb 15, 2016

fog37

Hello Forum,

It is said that astronauts on the International Space Station (ISS) experience microgravity and not complete zero gravity. The acceleration of gravity at the ISS altitude is still pretty significant (~8.7 m/s^2), far from zero.

However the free fall motion of the ISS and its passengers produces the sensation of zero weight (weightlessness), i.e. the apparent weight becomes zero but the actual weight is not zero. The sensation of our weight disappears.

Why is it called microgravity? Where does the tiny gravity effect come from? Shouldn't the free fall motion provide that complete zero gravity sensation and feeling?

thanks
fog37

2. Feb 15, 2016

Staff: Mentor

I assume that ≈350 km height isn't enough to be in a 'good' vacuum. ISS loses height over time, it has to be lifted from time to time.
Therefore it's not a perfect free fall. (Maybe also the moon's gravity comes into play.)

3. Feb 15, 2016

fog37

I just read that because the ISS is big, as big as a football field, there is a variation in gravitational acceleration much as 14 micro-g between the bottom and the top of the ISS, i.e. there is a gravity gradient.

I am not clear why this gravity gradient would cause microgravity. Aren't both the top and the bottom of the ISS free falling? at the same time? What does this micro gradient cause on an object that is inside the ISS?

Also, I would say that the microgravity could be caused by the mutual attraction between the various objects inside the space stations (astronaut to astronaut, space station to astronaut, etc.)

thanks

4. Feb 15, 2016

fog37

Thanks fresh_42. I guess you mean that because it slows down it start getting out of orbit (constant distance from the earth surface) so it needs to be lifted back at the right altitude and speed and that involves forces which break that zero gravity/free fall balance...

5. Feb 15, 2016

Staff: Mentor

But not at the same rate nor at the same gravitational acceleration. The top is further from earth than the bottom, so it is being accelerated less by gravity. And it is moving faster, so it is being accelerated more by centripetal acceleration.

Just a little.
I'll give you a hint: it's in the title of the thread and it also contains the word "micro"...

6. Feb 15, 2016

fog37

thanks russ_watters.

The bottom part of the ISS feels a downward acceleration of gravity g_bottom and the top part an acceleration g_top.

g_bottom > g_top

The ISS moves downward as a rigid body. The role of the centripetal force is played by the force of gravity mg itself. The bottom part seems to require a larger centripetal force than the top part of the ISS because it need to follow a more curved path than the top part. However, the top part travels at a faster tangential speed so it would seem to require a larger centripetal force....I am a little confused on which part (top or bottom) experiences the largest centripetal force...

And how would this create a microgravity environment on the objects inside the ISS?

7. Feb 15, 2016

fog37

8. Feb 15, 2016

Staff: Mentor

The ISS is rigid, so it is under tension due to these forces. Two people floating on opposite sides of the station are not rigidly connected: so they drift apart.

9. Feb 16, 2016

fog37

Thanks. I didn't know that (i.e. two people could drift apart).

So if an experiment is carried out inside the ISS, it should be carried as close as possible to the center of mass of the ISS where the microgravity is the least. Otherwise, the components of the experiment will feel these gravitational type of forces.

The point about experiments in microgravity is to eliminate all possible forces acting on the experiment (force of gravity included, as much as possible)

10. Feb 16, 2016

ZapperZ

Staff Emeritus
But if everything is "falling" at the same acceleration, there is no difference between that, and being in zero g.

Zz.

11. Feb 16, 2016

fog37

Thanks ZapperZ.

I wasn't clear. I guess I would say we can simulate the absence of the force of gravity by using free fall, i.e. making everything fall with the same acceleration (~8.7 m/s^2)

What interested me in this topic was the fact that it is not a perfectly zero gravity environment and wanted to know the causes of that...

12. Feb 16, 2016

ZapperZ

Staff Emeritus
But I still don't understand your problem.

You DO know that in a uniform circular motion, the object making that circular motion is in a constant "free fall" towards the center, don't you?

If ISS is in "zero g", then it will NOT move in a circular orbit! The fact that it is and not flying off to some random direction means that it is still tethered to the earth's gravitational field. Yet, I claim that this is the same as being "weightless", and that this weightlessness is no different than being in "zero g".

Zz.

13. Feb 16, 2016

fog37

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)

14. Feb 16, 2016

ZapperZ

Staff Emeritus
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.

15. Feb 16, 2016

fog37

Ok, let me try to be more clear:

1) The ISS experience a force of gravity downward equal to (M_iss)*(8.7 m/s). This force is far from being zero.
2) The force (M_iss)*(8.7 m/s) plays the role of the centripetal force
3) Weightlessness is the lack of the perception of weight, i.e. the lack of the sensation of this force (M_iss)*(8.7 m/s)
4) We produce weigthlessness, i.e. the sensation of weight and its effects too, by free falling around our planet
5) the point of making experiments on the iss is to produce conditions un which the objet being investigated does not feel the effects of gravity

16. Feb 16, 2016

ZapperZ

Staff Emeritus
But you haven't shown the difference between "zero g" and "weightlessness", and HOW, in terms of mechanics, that those two would be any different! If you are inside a closed box, can you construct an experiment to distinguish between the two, i.e. can you determine if you are really in "zero g" or just "free falling"?

Zz.

17. Feb 16, 2016

A.T.

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".

18. Feb 16, 2016

fog37

Ok,

I guess I should say that the feeling of zero gravity is due to the absence of a contact support force (normal force) on our body. The contact may be there but not the force itself...

19. Feb 16, 2016

ZapperZ

Staff Emeritus
You haven't answered my question. Can you device an experiment to distinguish the two?

Zz.

20. Feb 16, 2016

fog37

Well, the typical experiment is an elevator in free fall. We are inside the elevator with a scale under our feet. The scale reads zero. The"apparent weight" is zero even if our actual weight is still mg....

But that apparent weight being zero has real effects: our body parts don't feel the same type of compression it would if the elevator was not in free fall.

any better?