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Weight / normal force[edit]

Weight is either the force of gravity (second paragraph, which is the definition of weight elsewhere in Wikipedia and in most physics textbooks) or it is the normal force of the scale pushing up on you (first paragraph of Overview: Quote What we feel as "weight", is actually the normal reaction force of the ground (or whatever surface we are supported by) "pushing" upwards against us Endquote). It obviously cannot be both at the same time. Yes, people use the second definition in common speech -- but they come to the Wikipedia for clarification. "Apparent weight" is the correct, most widely-accepted term for the scale reading. Billt4 (talk) 23:51, 13 June 2008 (UTC)

Why this is complicated[edit]

The problem with the common use of 'weightless' is that the usual etymology doesn't work. Elsewhere, Wikipedia says 'In most physics textbooks, weight is the name given to the force on an object due to gravity.' Then we look at the suffix: 'legless' means without legs. 'hopeless' means without hope. So one would expect 'weightless' to mean without weight. Now the astronauts in the space station have roughly their normal weight: it is their weight that keeps them, and the station, in orbit. What would you say to beginning this article by saying something like:

'Weightless' is used in a somewhat illogical way. We might expect it to mean 'without weight'. In fact, it means 'without apparent weight'. So, to explain weightless, we must begin by explaining the difference between 'weight', which is the gravitational force exerted on an object by a nearby planet, star etc, and 'apparent weight', which is the force measured between and object and its near environment. Why are these different? The force measured by bathroom scales when you stand stationary on them is your apparent weight. It is not quite equal to the gravitational force that the earth exerts on you because, even though stationary with respect to the bathroom, you are rotating with the earth and therefore have a centripetal acceleration towards the earth's axis or rotation. — Preceding unsigned comment added by Inala (talkcontribs) 12:19, 24 March 2011 (UTC)

So, some months after adding these pars to the talk page, I've inserted a paragraph to deal with the contrasting meanings of weightlessness. Inala (talk) 10:52, 1 November 2011 (UTC)Inala

Why do you provide inaccurate information?[edit]

At low altitudes gravity decreases by 78 ppb per meter, centrifugal force increases by 157 ppb per meter, so jointly they give the change of -234 ppb per meter, which is -0.7 ppm per 3 meters. 21:23, 1 August 2007 (UTC)

Why are you using parts per billion and parts per million to refer to gravity? Gravity is expressed in units of force or units of mass per units of length, not units of density. – SarahTehCat (talk) 23:08, 15 June 2015 (UTC)

Suggested Microgravity experiements[edit]

A similar variant to the NASA's Vomit Comet Program is the ESA's A300 Zero-G Program.--ConradKilroy 16:30, 25 June 2006 (UTC)

The concept of weightlessness in an orbiting spacecraft is well explained and understood. One point which seems to be missed in this article is as follows: Does an astronaut experience weightlessness when the spacecraft is travelling in a straight line under its own inertia from the earth towards the moon? I shall be grateful if someone can throw light on this query. -- Dadakr 00:39, 17 December 2005 (UTC)

I thought that when using SI units it was unnecesary to put in these constants as the system is designed around them --sodium

SI doesn't eliminate the constant in this case. It does for F=ma, for instance (this is F=kma in other unit systems) but not for F=k/r^2. In this case k=-G M1 M2, where G is 'big G', the gravitational constant, and M1 and M2 are the masses of the gravitating bodies. -- DrBob

How does a wood block experience anything? -phma

Presumably it does so in ways that only another wood block could understand.

You are obviously taking an analogy (i.e. the woodblock) too seriously. This means that you are either very slow and stupid, or else you are so logical that the concept of metaphor is simply beyond the comprehension of your over-developed brain.

It would be preferable if you would avoid personal attacks in this forum. Especially when you can't manage to add your own signature. — RJH 17:53, 13 Apr 2005 (UTC)
Actually, I support Overlord359's comment. phma was being a jackass.

"Lesser symptoms [of weightlessness] include... weight loss...." You don't say! I know it's pedantic, but would "body-mass loss" or something be less stupid-sounding? 19:53, 4 January 2006 (UTC)

Hot air[edit]

How does hot or warm air from a heater or any other source of heat behave in a spaceship experiencing zero gravity?--Light current 03:05, 26 February 2006 (UTC)

Convection is the transfer of heat by mixing and flowing currents. Free Convection doesn't happen in microgravity, since there's no gravity to pull cold air, which is denser and less bouyant, down and get the currents moving. However, most heaters have fans that help them draw in and recirculate air (forced convection), so they'd probably continue to work, though not as well as they would on the ground.
Conduction is the direct transfer of heat when two molecules bump into eachother. Conduction does work in microgravity, but the one-molecule-at-a-time transfer of heat away from the source is much less efficient than convection, so heat builds up in the air around the heat source. You can get superheated pockets of air that way, which are a hazard space agencies have to be concerned about.
As an example, a lot of computer cooling systems work partly by free convection and have to be redesigned so the computers don't melt themselves. Corvi 00:12, 7 March 2006 (UTC)

On a related topic, what would a flame look like in 0 gravity? I think it may be spherical. It'd be hazardous to try it on one of our oxygen rich spacecraft, but it'd be interesting. (talk) 06:42, 24 January 2010 (UTC)

proposal to rename Vomit Comet to Reduced gravity aircraft[edit]

There is a proposal to rename Vomit Comet to Reduced gravity aircraft.
Please comment at Talk:Vomit Comet. --Jtir 20:58, 29 October 2006 (UTC)

Criticism of the terms "Zero Gravity" and "Microgravity"[edit]

this section should be deleted, or greatly reduced/altered —Preceding unsigned comment added by (talk)

You did delete this section, at 00:10, November 29, 2006, along with some other sections like "European Space Agency A-300 Zero-G". I'm restoring most of those changes. I agree that the "Criticism" section is too long and could make its point with fewer words. However, I think the criticism is valid -- "zero gravity" is a misleading term. I don't agree with completely deleting the section on the ESA program. Let's continue to discuss here. --Jdlh | Talk 20:17, 29 November 2006 (UTC)

Sorry about the ESA deletion, it was unintentional. The point in Criticism is valid, but I think it should be more of a footnote. As it stands, it is the lengthiest section of this article. The explanation sounds very original, and the editor used way too many "quotation marks" in my opinion. It reads like a rant. "For a weightless astronaut to say that they are in zero gravity is the same type of error as saying that an object that has a length of 0.3048 meters is identically one human foot in flesh and blood." should be removed. The editor makes the same point in each paragraph, which sounds redundant. The last paragraph should be removed. All in all, this section just doesn't sound encyclopedic. Just my opinion.

I agree, it does need to go. Not only is it too long, it offers no citations and sounds like all original research. While the criticisms do seem valid, it seems it is nothing more than one person's point of view. Kelvinator 21:33, 10 December 2006 (UTC)

This entire section is uncited, and without a citation it violates WP:NOR. If anyone wants it back in, please provide citations. Sethie 18:56, 25 December 2006 (UTC)

I am the one who added that Criticism section. I fail to see how anything I wrote qualifies as original research. The whole section was to point people back to the accuracy of very old research. My comments were based on the nature of gravity and acceleration that were published in 1687 by Isaac Newton. The reference is his Principia. The section talks about well proven knowledge that is over 300 years old. And I don't see anything new in discussing orbiting objects either. Newton's Principia discussed how the Moon orbits the Earth. The physical principles are the same. If the Moon were somehow stopped in its track, it would come crashing straight down into the Earth. Newton was well aware of this. I post it to Wikipedia and somehow it is seen as Original Research. I hope you can see that the entire section is essentially an explanation of Newton's ideas about gravity and acceleration. And a different way of explaining someone else's research does not in itself constitute new research. As for anyone who may still hold that the section was original, I offer this quote from the WP:NOR page:
"The original motivation for the NOR policy was to prevent people with personal theories attempting to use Wikipedia to draw attention to their ideas."
In contrast to this, the section at issue had the intent of drawing attention to the well-published and well-tested ideas of Isaac Newton.
There have been other complaints about the section also:
- It is too long,
- It is redundant, with the same point being made in each paragraph,
- Way too many quotation marks are used,
- It doesn't sound encyclopedic / it reads like a rant,
- It has no citations.
The length has a specific purpose in that most people hold misconceptions about weightlessness and this section strives to clear up such misunderstandings. That is not as easy a task as learning something from scratch. Added to this is that the topic is about an environment that people don't typically experience. They often have a lot of difficulty relating to it. Therefore much of the section length is spent in reframing the issue into situations that people CAN relate to, such as riding an elevator or watching an airplane fly.
As to redundancy, I agree that the point can be made with much fewer words. Ideally, the section would not be needed at all because there would be no misconception about weightlessness being "zero-gravity". But that is the very problem. There is a HUGE misconception. NASA itself promotes these erroneous views. To delete the section is to perpetuate the error. But Wikipedia serves to help us with the receding of the veil of ignorance. Newton did a huge effort toward that end back in the 1600's. My effort was simply to point back to a little piece of what Newton taught us, and what some of us have forgotten.
Quotations are used around terms that are being criticized as erroneous. Think of it as a quarantine of viral memes.
As to criticism that it doesn't sound encyclopedic, I am open to any and all improvements to the section. Particularly with any perception that "it reads like a rant".
Finally, the issue about citations has already been addressed with WP:NOR response above. Now if anyone would like to add the reference to Newton's Principia, I have no objection. But it is all just basic physics that is typically taught in late high school / early college.
I would like to see the section added back in, unless anyone has a substantial rebuttal to my response here.
Tdadamemd 23:54, 4 January 2007 (UTC)

Actually you haven't addressed the citations issue, you just.... ignored it.
Find a source which says, "Zero gravity/microgravity/weightlessness is an innacurate term," that will solve everything real quick.
I don't believe you can. I believe you are taking a bunch of different facts, which you could most certainly provide citations for, and then YOU are making all sorts of connections and conclusions between them. It's really, really simple- find a source that says exactly what you want to say, or leave it out. Sethie 01:02, 5 January 2007 (UTC)
Tdadamemd, thanks for your interest in this article and your contributions to Wikipedia. I share your criticism of how NASA and others use the term "zero gravity", but I think that Sethie is correct in saying that what we need is a published, verifiable source that makes that criticism. Then I'd support summarising the criticism that source makes, and citing that source. What you had here sounds like it was your own words, published here for the first time. That is what I think of as original research which the Wikipedia has a policy against. Does that make sense? --Jdlh | Talk 21:34, 5 January 2007 (UTC)
My God! Sometimes I am such a hard-ass. Great post Jdlh- you said the exact same thing, with such a more pleasant tone. Thanks. Sethie 08:06, 11 January 2007 (UTC)
No one has expressed support for my reply here, so to honor the criticisms I decided to repost the original Conflict section as a link external to Wikipedia. I hope everyone here sees that as a viable solution.
The section can be reincorporated when satisfactory references are found.
Tdadamemd 08:31, 11 January 2007 (UTC)
Cool- works for me! Sethie 08:33, 11 January 2007 (UTC)
Hey hey! Over at where the section was reposted, one of the veteran members offered a reference that was published in Omni magazine back in 1993 that makes the same type of criticism of the terms "zero gravity" and "microgravity". The author is none other than Jim Oberg, widely held as a leading authority on spaceflight.
Here is his contribution: Jim Oberg reference to his 1993 Omni article
I trust that this will more than suffice as consensus for meeting Wikipedia standards for getting the Criticism section reincorporated as part of the article.
Tdadamemd 03:04, 12 January 2007 (UTC)

That's fantastic that you found a source. If you want to incorporate the section, please write it using that article as the source, i.e. don't put in any ideas not found in that article.

Congrats!Sethie 03:40, 12 January 2007 (UTC)

Congratulations on a good find, Tdadamemd. Here are a couple of citations for you to use. Take the wiki source text; it uses templates that do the formatting and are easy to transplant. --Jdlh | Talk 22:44, 13 January 2007 (UTC)

Thanks for the help and support. With all that has come to light here, I'd like to take a step back myself for the time being. I'm sure that there are other Wikizens who will want to make improvements to this aspect of the article. I'll check back on it in a while.
Tdadamemd 19:45, 18 January 2007 (UTC)
Well, I'll take it on. The term 'Zero Gravity' is completely wrong. An astronaut on the Shuttle or ISS is subject to the gravity field of the Earth, which is after all what keeps them in orbit. For Wikipedia to blindly propagate sloppy science would be to make us part of the problem. Even if it's a widely-used term, if it's scientifically incorrect, we would be irresponsible to simply parrot the popular misuse The Monster 16:41, 4 March 2007 (UTC)
Argh -- okay okay, but please emphasise the astronauts truly are weightless, and all this talk about being attracted to the centre of the spacestation is about as relevant as the effects of large distant stars. There is only one bullet point in the article at the moment that is relevant, and that is the small amounts of atmosphere -- if the Earth had no air, you could be in orbit near sea level, or at least as low as Mount Everest. You'd just have to go even faster. But thanks to the edge of the atmosphere, anything in orbit will slow down slightly from the exact speed it needs to keep at that exact altitude. Whophd 09:37, 9 March 2007 (UTC)

Hang on, W=MG so where W=0, yet mass is constant (albeit with small variations) => W/M==0/M=G=0. Non? Alex (unregistered wikiuser whatchamacallit)

Wait a second. This article needs a broader perspective. Arguments about the inaccuracy of the terms "zero gravity" and "microgravity" are well-taken in general, but this article and free-fall are also linked to from articles discussing interplanetary and interstellar space. All the stuff about what "free-fall" means for objects in orbit is effectively irrelevant when you expand your horizons to interstellar travel. The article should focus primarily on orbital mechanics because that's the context of all spaceflight so far, but should not completely ignore situations not involving orbit around a planet, moon, or even the Sun. PubliusFL 19:40, 2 May 2007 (UTC)

I agree, 3 years later. I was more interested in what lack of gravity was like in deep space, not in orbit. This article has no information. (talk) 06:46, 24 January 2010 (UTC)

It is the same, but without microgravity.--Patrick (talk) 08:18, 24 January 2010 (UTC)

microgravity manufacturing[edit]

There must be a section on microgravity manufacturing, from perfect round ball bearings to perfect zero G pharmaceuticals. It's basically the biggest economic reason for a space station. JAF1970 18:53, 14 February 2007 (UTC)

Hey, if you can find the sources and edit the article, please go ahead and do so! It would be a fine addition. I could perhaps find some sources which claim that the microgravity manufacturing claims never had a strong economic base, and were only a cover for the true reasons for doing the space station or shuttle, which were national pride etc. --Jdlh | Talk 07:55, 16 February 2007 (UTC)
I have my hands full with other articles, but I'll try. JAF1970 07:09, 19 February 2007 (UTC)

The lead section[edit]

I pity the poor high school student with an assignment due who comes across this:

"Weightlessness is experienced by people during free-fall. Although the term 'zero gravity' is often used as a synonym, weightlessness in orbit is not the result of gravity itself being eliminated or even reduced significantly (in fact, the acceleration due to gravity at an altitude of 100 km is only 3% less than at the earth's surface . . "

For God's sake slow down! The article hasn't even established a firm grasp of weightlessness and free fall and given examples, before it's hitting the poor reader with provisos, exceptions, variations, the idea of orbiting, the concept of the accleration due to gravity . . . in a 64-word sentence containing at least five important ideas! It needs a rearrangement, give them some familiar examples to hold on to straight after the definition, make sure they get what free fall means -- it's too important to this article to send the reader off via that link to find out what it really means, it needs to be encapsulated in a sentence here, otherwise the lead section can't hold together. Then bring in orbiting and the problem with loose usage such as 'zero gravity'. Also, in terms of style all those brackets — and italics in the Overview — detract from readability. It may be correct, but is it digestible? A lead section should be "written in a clear, accessible style so as to invite a reading of the full article", and I think that unless the reader has already done physics in the last couple of years of high school, they are going to say: no, the effort of working out what this means outweighs the benefit I'm likely to get from it. Rexparry sydney 13:36, 17 August 2007 (UTC)

WP:SOFIXIT. — Swpbtalk|edits 15:35, 30 July 2007 (UTC) 13:41, 17 August 2007 (UTC)


Because zero gravity redirects here, linking to this page is under discussion on Wiktionary. I am not a Wikipedian so I am curious how it would be possible to link the other way around, from here to Wiktionary. Besides weightlessness, it would be possible to find definitions for zero gravity and possibly microgravity, with shades of meaning as the article notes. What is the Wiktionary template, and can it be used to define more than one term, if it is prudent to do so? (talk) 07:57, 20 March 2008 (UTC)

Hi, thanks for coming to help us at Wikipedia! There are templates to make links from here to Wiktionary; see Wikipedia:Wikimedia sister projects#Wiktionary. "Links to Wikimedia sister projects are best placed in the section of the article to which they relate, including Lead section, if possible...." I've added some links for "weightlessness", "zero gravity", and "microgravity". --Jdlh | Talk 19:05, 20 March 2008 (UTC)
Cool, thanks. I don't think I've never seen so many such links. Looks like I've got a new project now. (talk) 03:06, 23 March 2008 (UTC)

Rotating Asteroid[edit]

Another way of achieving "weightlessness" would be to stand on the equator of a spherical asteroid rotating at the correct angular speed. If your weight (i.e. your mass times the gravitational field strength) equalled the centripetal force needed to keep you "orbiting" the asteroid's centre, you would feel "weightless". [To analyse this situation, we can use Newton's Second Law, i.e. net force = mass times acceleration. The net force is mg-R (where R is the reaction force exerted upwards by the ground on the person's feet) and the acceleration is r times (omega squared). Weightlessness happens when R is zero, i.e. when mg = mr times (omega squared); g = r times (omega squared). ] If we slightly slowed down the asteroid's rotation, and built a skyscraper on the asteroid's equator, we would find that one's weight would be downwards at the foot of the skyscraper, but one's "weight" would feel "upwards" (i.e. R would be negative) at the top of the skyscraper. In a room at the bottom of the skyscraper, chairs would stand on the floor, but in a room at the top of the skyscraper, chairs would be upside down and standing on the ceiling. If the skyscraper was tall enough, the skyscraper itself would be "weightless". Hence the idea of the very tall (i.e. thousands of miles tall) tower on the Earth, so tall that it would be "weightless", and acting as a "space elevator". These sort of calculations reveal that there is a limit to how rapidly an astronomical body can spin; if an astronomical body spun too fast, it would disintegrate because gravitational forces would not be powerful enough to provide the centripetal forces necessary to keep the material of the body rotating. [Martin] —Preceding unsigned comment added by (talk) 19:14, 23 March 2008 (UTC)

Martin: Interesting! I hope you can find someplace outside of Wikipedia to publish this. Unfortunately, it's not suitable for Wikipedia as-is because it looks like original research, which we don't do; we just cite other work that was published by reliable sources. --Jdlh | Talk 20:49, 23 March 2008 (UTC)


W=mg, All forces cancel each other in weightlessness. This means mg=mg=mg=....... when g = 0, then also m=0 . This means mass should be disappeared so is it possible in zero gravity for a mass to be zero.

Just a formal discussion not a denigration. Myktk (talk) 03:47, 21 October 2008 (UTC) Khattak

No, mass is never equal to zero. Mass, in fact, does not change at all - changes in weight (a force) are entirely due to changes in gravitational acceleration (or to look at it another way, changes in acceleration are entirely due to changes in the applied force of gravity). The only instances in which mass changes are when relativistic effects are considered, and these effects are only significant for objects moving at incredibly high speeds (near the speed of light) or in extremely strong gravitational fields (such as near a black hole).
One can tell that objects in zero gravity still have mass, even though they don't have weight, because they still possess momentum. If a floating astronaut were to catch a thrown ball, he would be accelerated in the direction the ball was moving - the ball would transfer energy to him. In order to have kinetic energy, the ball must have both a velocity and a mass. — Swpbτ c 14:03, 21 October 2008 (UTC)

Another prime example of gravity effects in an orbital microgravity environment[edit]

Template:Tleditsemiprotected additional example of gravitational effects in microgravity:

  • "Floating" objects in the Space Shuttle are actually in independent orbits around the earth. If two objects are placed side-by-side (relative to their direction of motion) they will be orbiting the earth in different orbital planes. Since all orbital planes pass through the center of the earth, any two orbital planes intersect along a line. Therefore two objects placed side-by-side (at any distance apart) will come together after one quarter of a revolution. If they are placed so they miss each other, they will oscillate past each other twice per orbit. If they are placed one ahead of the other in the same orbital plane, they will maintain their separation. If they are placed one above the other (at different radii from the center of the earth) they will have different potential energies, so the size, eccentricity, and period of their orbits will be different, causing them to move in a complex looping pattern relative to each other.

Reference: Chandler, David, "Weightlessness and Microgravity", The Physics Teacher, May 1991, pp. 312-13

AEDC (talk) 07:02, 24 November 2008 (UTC)

Yes check.svg Done MSGJ 11:17, 24 November 2008 (UTC)

Effects on non-human organisms, other sources[edit]

In the article, in the citation for the statement, "Fowl eggs which are fertilized in microgravity may not develop properly", there was the following comment:

 NB: May or may not support above egg statement.
 A Preliminary Biophysical Report on the Fertilized Eggs Traveled with Spaceflight

I think the contributor means that the article in may or may not be a further source for the claim about fowl eggs. Perhaps they were offering it as a trail for other editors to follow. If I understand correctly, then I think a comment in a ref.../ref tag in the main article isn't the place for it. It should be a comment here on the Talk page, for someone to read and perhaps decide to investigate. So I've moved it. --Jdlh | Talk 06:12, 12 January 2009 (UTC)

What is zero gravity?[edit]

An object can be weightless if it is held in place by gravity - if gravity is balanced out by another force (inertial or centrifugal force), then there is ZERO GRAVITY. Am I right? Majopius (talk) 01:48, 2 April 2009 (UTC)

Low energy free fall trajectories are ellipses, not parabolas[edit]

Any object on a true parabolic trajectory would be travelling at nearly 25,000 miles per hour and would promptly escape the earth. [William Tyrrell Thomson, Introduction to Space Dynamics, Dover 1986. p. 91.]

This is a pervasive misconception, even the pilots call them parabolas. —Preceding unsigned comment added by Norbeck (talkcontribs) 03:10, 30 December 2009 (UTC)

In a model with constant gravity free fall trajectories are parabolas.--Patrick (talk) 23:51, 21 January 2010 (UTC)

Zero-g does not mean zero gravity[edit]

Zero-g (lowercase g) is short for zero (local) acceleration. The "g" in this case is a unit of acceleration (standard gravity "acceleration equivalent to that caused by gravity while on the surface of the earth") not a unit of gravity (despite the confusing name). There is a common misconception the Zero-g means zero gravity and is therefore and incorrect term to describe being in orbit. But, in fact zero-g (meaning no acceleration) is quite accurate. I see two problems with the current page. The first is that zero-g redirects to this page. The second is that this page does not mention zero-g. It does how ever mention "zero gravity". In fact it has a whole (correct) section on why zero gravity is a misnomer. But nowhere does it mention that zero-g means zero acceleration. This leaves the user with the impression that zero-g is short for zero gravity. Stephen Luce (talk) 01:23, 20 February 2010 (UTC)

Well, zero-g is actually much better understood as zero-g-force. There is a whole article on g-force, which is defined as proper acceleration, not coordinate acceleration (dv/dt). The reason this makes a difference is that there are many zero-g environments in which there is no proper acceleration, and thus no g-force, but in which there is certainly coordinate acceleration. Astronauts in orbit and people in falling elevators are certainly accelerating, by the coordinate definition of acceleration. They just don't feel it, because they are in inertial trajectory, or free-fall. Acceleration per se is not the key, but rather that acceleration away from that of an inertial or free-fall path (or the coordinate acceleration of an inertial path). And by the way, Einstein would remind you of two things: 1) you can't feel gravitational forces anyway (even standing on Earth, you feel the Earth) and 2) that when you fall in an elevator or in orbit, the g-field actually does disappear FOR YOU, with the exception of tides. So zero-g is a perfectly good term for orbit, if you're talking about the inertial frame (gravity re-appears when looking at it from the ground). Micro-g is a better term for what you "see" in orbit. SBHarris 02:36, 20 February 2010 (UTC)
I agree with your point about proper acceleration vs. coordinate acceleration. I could have been more specific. The point I was trying to make is we should try to explain that zero-g is not an abbreviation for zero gravity and micro-g is not an abbreviation for microgravity. The "g" in these terms stands for acceleration, not gravity. I feel that this distinction should be made on the page that you get to when you wiki search for "zero-g". I think it should also be pointed out on the Micro-g environment page. Actually on that page an attempt is made to explain the term. But it could be a little more clear about it. Stephen Luce (talk) 21:27, 20 February 2010 (UTC)
But how to explain this in the lede? Per Einstein, it is perfectly correct to refer to conditions in orbit as "zero gravity" (approximate) or "micro gravity" (more precise). In relativity, gravitation disappears for (local point-like) intertial observers-- it's not just that you don't feel it, it's that it does not exist. In Newton's view, you simply don't feel it, as usual, BUT also no longer feel any resistance to it (which is why you don't feel "weight," which is a thing that arises from other feel-able forces). So you are weightless. Same practical result, which is nice.

Now, it is perfectly true that the term "zero-g" originally MEANT zero-proper acceleration (which is traditionally measured for astronauts in "g's," because it is proper acceleration which produces the feelable and dangerous effects). All this is discussed in the article on g-force, a misnomer which actually is about the proper accelerations measured in units of g, and which is the thing which goes away (goes to zero) in weightlessness. BUT, it all turns out okay in the end, because zero-g in both senses (which are different) is still true for inertial observers. If we have to go into that, however, it should be in the body of the article. And I'm afraid will cause the scratching of not a few heads. SBHarris 22:09, 20 February 2010 (UTC)

Somebody keeps reverting the fact that gravity is not felt as a force[edit]

The "force" of gravity is not felt, period. That's not only true in free fall, it's true all the time. When you're standing on the ground, you do not feel the force of gravity, you feel the ground mechanically pushing up on your shoes, and THAT IS ALL. Remove the ground, and you feel weightless, which shows that you never did feel the gravity "force" component-- all you ever felt was the ground-force. Whether gravity is a "force", or what kind of force, we can leave to Newton vs. Einstein. Clearly, it appears as a force only in an accelerated frame (with the acceleration defined as proper acceleration), so the gravitational "force" is very much like a fictitious force-- one that appears only in accelerated frames, like standing on the ground. However, what is the same for both Newton and Einstein is that MACRO-gravity cannot be detected by feel or instrument. Only tides (microgravity) can be detected. A man in a falling elevator or in orbit cannot tell (except for tides) that he's not out in space away from all gravity fields. Nor can his instruments tell him. That's the point of this article. So stop reverting this, or else discuss it here, and I'll be glad to set you right. SBHarris 19:45, 30 October 2010 (UTC)

Our body does feel gravity. Every single cell of our body is compressed by two opposite forces, one from above (simplifying), which we call weigth, and one from below which is the necessary reaction to it. The effects of these forces are apparent also in inanimate object and you can see and measure them, by measuring the curvature of a wooden beam in a building, for example. Architects draw a lot of force lines to calculate the stability of pillars and beams, by equilibrating weigths and reactions. Things cannot be described otherwise, because 3rd law is not an optional thing. Gravity is a distributed, "diluted force", applied to each cell, to each atom, but is a force. And must follow the laws of dynamics. When you float in water, you almost do not feelo the reaction from water, because the force is distributed on an area much greater than that of your feet (or of the few cm2 of your heels, when you are standing), producing a much smaller pressure. This is the reason you feel mainly your feet, when standing: far from the feet, the force is distributed, "diluted", so you do not feel it so much, just like when floating. Besides being distributed, gravity has another peculiar thing: it is strictly proportional to mass, and this make it possible to have weightlessness in a falling elevator, and to curve space, and to make bodies to follow geodetics of curved space, and so on. But on Earth it is useless to complicate things speaking of Einstein. Newton is all we need to explain standing, sitting, floating, going up and down in elevators, falling with them, and orbiting, and, I say it again, since weight is a force, it cannot escape obeying the same three simple laws obeyed by all forces. --GianniG46 (talk) 18:17, 1 November 2010 (UTC)
See the article on fictitious force. Fictitious forces are "real" but they are not "felt." They are merely INVOKED in accelerated frames to explain perceived motion under Newton's laws. An "inertial force" is not a force in the normal way we think of forces (for one thing, inertial forces have no reaction forces, ala Newton's second law!).

Example: do a thought-experiment. Suppose you're on board an accelerating rocket, in deep space. In the rocket frame, it appears that you have "weight" due to the acceleration from the motor. How many forces are acting upon you? Just one force in the usual sense-- from the floor. And it has a reaction force from your feet per the second law, of course, but that doesn't make it two forces. There is an additonal one, but it appears and disappears, depending on the frame. It appears in the rocket frame, but disappears in the free fall frame (inertial frame). And it's never directly "felt"-- only assumed. All you ever "feel" is the floor! SBHarris 21:10, 1 November 2010 (UTC)

We are speaking on whether real forces, like gravity, are felt or not on an inertial frame like, approximately, Earth. And no doubt that in inertial frames Newton's laws work without invoking fictitious things. --GianniG46 (talk) 23:06, 1 November 2010 (UTC)

But that is the point. When you stand on the surface of the Earth, you are not in an inertial frame (inertial frame = free fall frame = float frame). Rather, you are in an accelerated frame, and that acceleration is due to the ground pushing up on you. "Gravity" is what you postulate to explain why you're not going anywhere in space (i.e., why you're not undergoing coordinate acceleration). The surface of the Earth pushing on your shoes does not cause you to undergo a 3-D coordinate acceleration, but it does cause you to undergo a proper acceleration in space-time. And that is what your accelerometer feels.

Remove the floor and now you are inertial. And your accelerometer reads "zero" also (since it doesn't read coordinate acceleration, but only proper acceleration). What you feel, is only forces that cause proper acceleration, which is acceleration away from an inertial path (which is free-fall toward the center of the Earth, in this case). So you don't ever feel gravity, either standing on the Earth, or in free fall down an elevator shaft. It is very much like the inertial force that appears in a rocket, but which appears only in the accelerated frame. If you free-fall on Earth THEN gravity disappears, but it was never "felt" in any case (even when you thought it was present). On Earth the (properly) accelerated frame is the one in which you are standing still on the ground. The non-accelerated frame on Earth (no proper acceleration) is free fall down a hole.

Again, it will help enormously if you familiarize yourself with the difference between coordinate acceleration (dv/dt= d^2x/dt^2) and proper acceleration. Sometimes they are the same thing (like out in flat space), sometimes (as on Earth) they are totally different. But proper acceleration (measured in g-force units) and the mechanical forces that cause it, are all you ever feel. SBHarris 02:18, 2 November 2010 (UTC)

So, I have to understand that Newton wrote his laws for the frame of the falling apple, not for his own frame. I repeat again: in classical, Newtonian mechanics, which is perfectly valid on Earth, the inertial frame was that of Newton sitting under the tree (neglecting the small accelerations arising from the rotation of Earth and the revolutions of Earth, Sun and Galaxy). Weight was the force, and the apple fell, accelerating toward Newton's head, responding to this force. According to you, instead, Newton's laws are useless on Earth, and we have to say: the whole Earth accelerated toward the apple, freed from its stalk, and Newton's head hit the apple. The problem is: the apple is free falling toward Earth, but Earth, too, is free falling toward the Sun, and Sun toward the center of Galaxy, and Galaxy toward the center of gravity of the Local Group of Galaxies, and so on. These are all inertial reference frames, according to you? Who has to familiarize with mechanics? --GianniG46 (talk) 08:53, 2 November 2010 (UTC)

By invitation by GianniG46:
The difference between the two viewpoints seems to be that GianniG46 insists that gravity is a "real force" (see [1]), whereas Sbharris's view is that gravity can be modeled as a force or as spacetime curvature, and that whatever it really is, we just call it gravity (see [2]). I'm strongly inclined to follow the latter viewpoint, since it is not only broader than the former, but it is also the modern view. I have moved the wikilinks to gravity and force to their first occurrences in the lead, in which these wikilinks were i.m.o. painfully missing.

So I strongly prefer the current, more balanced version of the paragraph. DVdm (talk) 10:08, 2 November 2010 (UTC)

The question is not at all semantic, whether one prefers to say force or curved space. The paragraph does not say "In general relativity one does not speaks of gravitational acceleration, but of curved space, so that in any case the trajectories are the geodetics of space-time". It says: we do not feel gravity, we feel only our feet. Since Newtonian mechanics must (approximately) be correct, on Earth, I have used it (third law) to explain that it is impossible to compress a cell of our body without appliyng two forces which oppose one another. Which is the way used by engineers to calculate the stresses and the deformations of beams and pillars when they build a skyscraper. I believe they use Newtonian mechanics, not general relativity, and nevertheless usually skyscrapers stand stable on their feet, which demonstrate that Newtonian mechanics work on Earth. So my problem is only experimental facts, not preferences between classic and modern. This fact, that each cell of our body and each element of a building are stretched and strained, surely can be described also with reference to general relativity, but I believe that it would be much less clear.--GianniG46 (talk) 14:58, 2 November 2010 (UTC)

Let me try again, without some of the complications. Whether from Newton or Einstein's view, we still don't feel gravity-- we only feel the floor, pushing upward. Newton and Einstein disagree on whether there is (Newton) a downward "force of gravity" acting on us, or (Einstein) an "inertia-like fictious force" that is reacting to the fact that the upward-pushing floor is mechanically dragging us on a non-geodesic through spacetime.

But all those stresses and strains you're talking about on your cells are explained just as well by only one force, and would be the same if you were riding a rocket. In that case, if you were outside the rocket, you'd attribute them to one force, but if you were inside, you'd need an oppositely acting inertial force to explain why you're not moving! But the squishing happpens even without the inertial force, because even in the non-rocket frame (as you watch it go past you), the cells of the astronaut still get squished, and you explain that via one single transmitted force from the floor. And no other!

The other point is that when you fall on Earth, even if you insist on Newton's view, the force of gravity is still there, but you don't feel it (no squishing). In Einstein's view it actually disappears (no squishing). But either way, no squishing. So from either view, you don't feel gravity in free fall, either. The basic take home message is you don't feel gravity, no matter how you view it. You never feel it. The squishing is always from the mechanical force of the floor.

A last comment: engineers don't necessarily use the view of two forces. One can, if one wishes, assume there is an (undetectable) force called "weight" pushing every object (building) downward which must be counteracted by a push from the ground. But it's even simpler (and the math is the same) to simply say it takes an W=mg force from the ground to keep every object from falling (which "free-fall" is its natural motion-- Newton says due to gravity-force, Einstein says due to curved spacetime). And that opposing force from the ground, to keep things still, is mechanical and thus always provides stresses in objects. And that's it. Again, just one force is felt. The other (the downward one) is in a sense only in your mind, but in any case, is never felt. SBHarris 20:01, 2 November 2010 (UTC)

Of course this discussion could be ended (aka solved) by finding some good source... DVdm (talk) 22:21, 2 November 2010 (UTC)

Any good discussion of the equivalence principle includes all this. [3] What I don't really understand is how anybody can argue that we DO feel gravity. When you're falling down an elevator shaft, you don't feel gravity and thus are weightless. When the elevator floor prevents this, what you feel is the force from the elevator floor, acting throughout your body. Which is the sensation of weight. What could be simpler? SBHarris 22:36, 2 November 2010 (UTC)
I fully agree, but a good textbook source (not a pointer to another wiki article) could make the discussion moot... DVdm (talk) 22:44, 2 November 2010 (UTC)
Yes, but alas we have come to one of the real problems on Wikipedia. Many intermediate textbook authors are confused on the subject themselves. If you really want the straight dope you have to go to a text on relativity, and then people complain that you've overcomplicated the subject. Well, the simple explanation of weightlessness is that "weight" is due purely to mechanical forces, and whenever you remove those, you are weightless. End. Actually one doesn't even have to discuss "gravity," except to note that even when gravitation is involved, you can't ever "feel" it (except as tides). I can't believe the amount of dithering over this matter, even in texts.

Incidentally, accelerometers can't feel gravity, either. Put an accelerometer on the ground, and it will register acceleration of 1 g, upward. That is caused by the push from the ground. That is all. SBHarris 23:09, 2 November 2010 (UTC)

It is useless here to mention relativity. Weightlessness is perfectly explainable by Newton laws. Were it not, we would not need to rely on delicate measurements of the peryhelion of Mercury, or of gravitational redshifts, to prove general relativity (see Tests of general relativity). So let us speak of classical mechanics.
And, in classical mechanics, weight is a force. And a force cannot be felt if it is alone, because it has no reactions, and third law says that every force must have an opposite force. So we do not feel it when free falling. When we are standing on our feet, instead, we have also another force, the reaction from soil. So we have two forces, weight and the reaction to it. There is no reason why we should feel only one of these two forces. The forces obey the same laws: so, if I feel one of them I must feel also the other, in one way or another. Weight is more "diluted" then the reaction, because it is applied uniformly to all the body, so maybe I feel it less than the other, but I must feel it.
But, to answer to your statement "the squishing is always from the mechanical force of the floor", that is from just one force, let us for a moment put apart gravity and go in deep space. Let us speak of other forces (muscular, for example). Can you explain me how I can squish an object with just one force? --GianniG46 (talk) 00:08, 3 November 2010 (UTC)

"Can you explain me how I can squish an object with just one force?" Gravitational waves ? Also Newtonian gravity and GR are different in other strange ways: in NG - pressure is a repulsive force, but in GR - pressure adds to the mass-energy density origin of gravity (Stress-energy tensor). You do feel pressure therefore you do feel gravity in GR (at least according to the theory) Einstein's Gravity Under Pressure. (talk) 09:58, 3 November 2010 (UTC)
A gravitational wave is not one force. It gives an oscillating field of forces, much like a sound wave, which pushes (in the wave maximum) and pulls (in the minimum) matter. --GianniG46 (talk) 10:24, 3 November 2010 (UTC)

Strictly speaking, whether we "feel" gravity on the surface of the Earth is not relevant to an article on weightlessness. I suggest we just avoid the issue, at least in the lead section. I have rewritten the lead section, adding some material on physiology. RockMagnetist (talk) 00:16, 3 November 2010 (UTC)

It's relevant to why you "feel" weightlessness in an elevator shaft, when all the happened was that the floor-force was removed due to the cable being snapped (no Otis brakes). You do not feel gravity when you fall. So why did you feel it when you felt the floor which was supported by the cable? Or did you just feel the floor? The second explanation is simpler, and the first one begs the queation of why you feel the force of gravity on your body with a floor under you, but not when the floor is not under you (or not pushing on you?).SBHarris 07:45, 4 November 2010 (UTC)
SBHarris, I lean towards your point of view on this issue, but I agree with DVdm - material that is challenged and not currently supported by an authoritative citation should not be in the article. I'm afraid the burden of evidence lies with you. Note also that the word "feel" refers to sensations, so a complete discussion would include physiological factors like the effect on blood circulation. RockMagnetist (talk) 13:46, 4 November 2010 (UTC)
See also WP:SOURCES. RockMagnetist (talk) 13:49, 4 November 2010 (UTC)

To answer GianniG46, an object out in far space can be pushed with any mechanical force until it collapses. That's one force. Sure, the object pushes back with a counterforce, but that is due to the object's inertia. Even though you may consider this force to derive from inertia, The object does not FEEL its inertial force! If it wasn't pushed, it would feel nothing (the inertial force just appears out of nowhere, when it is pushed). Gravity is much the same kind of thing, except it's an inertial force that appears out of nowhere, when you prevent an object from following a geodesic path. So it's no coincidence that the force from gravity is the same as the force from inertia, and the gravitational mass is the same as the intertial mass (something that had no explanation before Einstein). The reason they two kinds of masses are the same, is that they're the same mass and its the same process. The elevator floor or the rocket floor, is dragging you through curved space in both cases. Inertia resists in both cases. But you don't feel inertia in both cases-- all you feel is the accelerating force. "Feel" means inertia of one atom has to be overcome by another atom pushing on it. There is no "new" force pushing back-- just the old resistance to going forward. Which was never noticed before the push.

If you tug an astronaut through space by moving a mass close to him and then using it to tug him along, never letting them touch, that won't be felt. He'll be weightless all the time. Put him in a ship with no windows, and he can't tell a thing. SBHarris 07:45, 4 November 2010 (UTC)

I am wondering if we are understanding with each other, so I write here a simple questionary. Please answer with very few words to each question, preferably with yes-no, so we can find where is the disagreement.
  • 1) Must classical and relativistic theories give approximately (to a high level of accuracy) the same experimental results on Earth, so that I can equally well describe things in one way or another ?
  • 2) In classical mechanics is Earth approximately (that is, neglecting small centrifugal and tidal forces) an inertial frame of reference (i.e. in which Newton's laws are approximately valid)?
  • 3) On Earth, according to Newtonian mechanics, is weight a force?
  • 4) Is it true that, in inertial frames of reference, forces on resting objects have to balance one another (3rd law)?
  • 5) Are the cells of my heel compressed when I am standing (my weight is a substantial one)?
  • 6) Is this compression felt by me?
  • 7) If a cell of my body is compressed, and does not move, must it have (at least) two forces that oppose one another?
That's all. Please answer the questionary, so perhaps we can spot the problem. --GianniG46 (talk) 15:38, 4 November 2010 (UTC)
I think that this discusssion should not be had here per wp:TPG. It does not really matter what we think is true. What matters is that we find a source and reflect what is said in it. So why don't we look for a good source and see if we can agree on that? DVdm (talk) 15:48, 4 November 2010 (UTC)
It would be too easy for me to say that my source is Newton's Principia. That is why I have written the questionary, to understand where is precisely the problem. --GianniG46 (talk) 16:11, 4 November 2010 (UTC)
Hm... of all the possible sources that are compatible with the non-Aristotelian view, I'm inclined to say that Newton, being the first source, must be just about the worst possible source one could use in the context of modern physics, just like Einstein would be a bad source for matters like for instance "relativistic mass". They paved the way, and they must of course be cited in historical matters, but views and ways of explaining things have been drastically modernised since then.
Anyway... a Google books search with a few well placed keywords reveals stuff lke this (the Newton view) and this (the Einstein view). So why don't we just use these two sources and present both viewpoints? DVdm (talk) 20:04, 4 November 2010 (UTC)

Tangential flight path[edit]

"The orbit is maintained by the object's inertia tangential to its flight path and its acceleration towards the center of the Earth". - Kbrose, can you give me an example of a flight path which is not tangential to its own orbit? Even a car moves tangential to its orbit, or trajectory. And can you explain me how can the orbit be tangential to the acceleration? And what does it mean "that's exactly the point"?. --GianniG46 (talk) 22:02, 20 November 2010 (UTC)

Your interpretation is not what the sentence states. It's clearly talking about the inertia (...maintained by the object's inertia...) and gravity maintaining the orbit. A curved orbit require two forces to maintain it, not just one. The sentence's sub-clause states that the inertia is tangential to the flight path. Kbrose (talk) 22:21, 20 November 2010 (UTC)
The inertia vector (actually the velocity and momentum vectors, since I don't even know if you can express "inertia" as a vector) is always tangential to the flight path-- the sentence above doesn't suggest otherwise, although we could add an "always" so it's clear that this is ever the case. The orbital path is tangential to the acceleration (i.e., the velocity and acceleration are at right angles) in a circular orbit. But in all others, this only happens at four points in the ellipse. SBHarris 22:17, 20 November 2010 (UTC)
A straight line is not typically said to have a tangent, only curved paths that are differentiable at a point of interest are said to have a tangent line. Here, the sub-clause obviously simply serves to characterize the direction of the linear velocity of the object. Kbrose (talk) 22:31, 20 November 2010 (UTC)

I think that a sentence like "The orbit is maintained by the object's inertia tangential to its flight path and its acceleration towards the center of the Earth" needs a source. I have tagged it. On the other hand I'm sure that there must be a better (and properly sourced) way to say something about what maintains the orbit. DVdm (talk) 22:45, 20 November 2010 (UTC)

Two obvious statements: a) the tangent to a straight line is the line itself. b) acceleration and velocity may form any angle: zero (i.e. they are tangential) for a resting body which falls in a constant acceleration, pi/2 for a perfectly circular orbit, variable for an elliptical orbit. --GianniG46 (talk) 23:10, 20 November 2010 (UTC)

Personal website as a source[edit]

User Incompetence (talk · contribs) used this website as a source for an addition. I have replaced the source with a {{cn}} tag. Let's source this properly please, not with someone's personal website. DVdm (talk) 23:37, 20 November 2010 (UTC)

Reduced weight in pilot training[edit]

Just about to zap that section. Reason being that its content is made up--there is sort of a reference, pointing to a gliding club website in New Zealand, which currently results in a 404. In any event, that would have been some of the "fun" training/activities that clubs do.

This is in no way part of the pilot training syllabus anywhere that I'm aware of. It is certainly not mentioned in the JARs (if anyone can find a link to EASA-FCL, assuming it even exists yet, much welcome), and neither is in the FARs.

So in summary, that section provides incorrect information and therefore it goes. — Preceding unsigned comment added by (talk) 23:37, 31 May 2011 (UTC)

The new lead is too short and is also wrong[edit]

It is too short and does not summarize the article. In addition my opinion is that it's flatly wrong. The meaning of weightlessness in physics is not greatly different from ordinary language, which is absence of weight. Weight being the force that acts between you and your bathroom scale and between you and whatever else supports you or that you feel as a force (All forces acting equally between two bodies per Newton's third law.) When that force is gone, so the scale reads zero, and you feel no others pushing or pulling at you, then you are weightless. That is enough for a lede and everybody agrees with it.

Problems enter in when we try to give a mechanism for loss of that force, which loss is what we feel, when weightless. And there's an additional unnecessary problem that comes in when you choose a minority definition of weight as always being mg, but that just points up the problems of this bad gravitational definition, not the problems with "ordinary language" which doesn't use it. Or the difference between ordinary language and physics, which doesn't use it all the time, either, and probably should not use it at all. Not least because it is malpredictive of what happens between you and your bathroom scale except in the very special case that you are not changing velocity. So let us not complicate this. The ordinary "instrumental" view of weightlessness works very well in all situations. Other views can certainly be kept out of the lede as being ornamental and/or outdated. SBHarris 00:58, 25 November 2012 (UTC)

The current lead is short and sweet and not wrong[edit]

Hello and thank you SBHarris for your comments above. The lead is not a summary of the article but that is no bad thing in this instance. It warns the reader who is not much delayed from the meat of the article. My main disagreement with you however is that you claim the gravitational definition of weight, weight1 in the article, to be the minority definition. I claim,on the contrary, that it is the dominant definition. On the other hand, the dominant sense of weightlessness in physics is to do with the absence of operational weight, instrumental weight, weight2. The physicist and the woman on the Clapham bus may both say, "Weightlessness is the absence of weight" but my point is that what they mean is not the same. Interestingly, this whole confusion about weightlessness arising from the ambiguities of the term weight would be much helped by a reading of the article "Weight" in wikipedia. Shanker Pur (talk) 10:52, 26 November 2012 (UTC)


This article should be merged with Microgravity as they're exactly the same thing. You're *never* weightless, only being affected by a super-small amount of gravitational force, or microgravity.

Thoughts? – SarahTehCat (talk) 23:17, 15 June 2015 (UTC)