# Help my coworker understand gravity in space **URL:** \ **Category:** Factual Questions\ **Created:** [November 6, 2009, 3:46pm UTC](https://boards.straightdope.com/t/help-my-coworker-understand-gravity-in-space/516581 "2009-11-06T15:46:51Z")\ **Posts on this page:** 20\ **Page:** 4
**Author:** ![CookingWithGas](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/cookingwithgas/32/485_2.png) [@CookingWithGas](https://boards.straightdope.com/u/CookingWithGas)\ **Post date:** [November 7, 2009, 3:29am UTC](https://boards.straightdope.com/t/help-my-coworker-understand-gravity-in-space/516581/61 "2009-11-07T03:29:03Z")
> [@Yumblie](#): > > Weight isn’t the force of gravity on an object. That’s simply Newton’s law of gravitation, F=Gm[sub]1[/sub]m[sub]2[/sub]/r[sup]2[/sup]. Weight is the force required to keep you from falling through the ground. If you’re not standing on the ground, you have no weight. I think you’re a little late to the party on this. See many posts above debating this point. Bottom line is it is a matter of rigorous scientific definition vs. popular definition. > [@Ponderoid](#): > > Gravity does not act equally on everything. Objects closer to a planet are pulled more strongly to it, objects further away are pulled less strongly, and these differentials are called tidal forces. There are microgravity experiments that can tell you whether or not you’re near a significant mass. By repeating the experiments over time, you’ll be able to tell whether you’re inbound, outbound, or in a stable orbit. If the tidal forces keep getting steadily stronger… Say your prayers. 🙂 I would be interested in what experiments these might be. I agree completely with your first two sentences, and they do not contradict what I said. But what experiments can tell you your location with respect to an arbitrary mass? I am not refuting this because I have just pretty basic knowledge of this area but I can’t imagine what it might be. ---
**Author:** ![Xema](https://avatars.discourse-cdn.com/v4/letter/x/9de053/32.png) [@Xema](https://boards.straightdope.com/u/Xema)\ **Post date:** [November 7, 2009, 5:55am UTC](https://boards.straightdope.com/t/help-my-coworker-understand-gravity-in-space/516581/62 "2009-11-07T05:55:56Z")
> [@CookingWithGas](#): > > But what experiments can tell you your location with respect to an arbitrary mass? I think the notion is to measure the difference in gravitation over a smallish distance. This difference will be larger near a significant mass than when far away from one. ---
**Author:** ![Nancarrow](https://avatars.discourse-cdn.com/v4/letter/n/c37758/32.png) [@Nancarrow](https://boards.straightdope.com/u/Nancarrow)\ **Post date:** [November 7, 2009, 6:24pm UTC](https://boards.straightdope.com/t/help-my-coworker-understand-gravity-in-space/516581/63 "2009-11-07T18:24:02Z")
> [@gazpacho](#): > > I read those links and they are talking about how your are weightless in free fall. Jesus Tittyfucking Christ. Yes, they do use the term weightless. Ill-advisedly, but whatchagonna do. Now look at the diagrams. And the big equation that says W = m g What definition of weight is used in the diagrams and equation? The one I’ve been banging on about. The useful one. The one that helps clear confusion on this subject. Most of the time. Fuck me, they only use the term “weightless” once AND THEY PUT IT IN SCARE QUOTES. Am I ever again going to look at a bowl of one of my favourite soups in the same way? And **Yumblie** , please do follow **CookingWithGas** ’s advice. And check out the links I referred to, particularly the NASA one. Now I damn well AM done here. ---
**Author:** ![CookingWithGas](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/cookingwithgas/32/485_2.png) [@CookingWithGas](https://boards.straightdope.com/u/CookingWithGas)\ **Post date:** [November 7, 2009, 7:16pm UTC](https://boards.straightdope.com/t/help-my-coworker-understand-gravity-in-space/516581/64 "2009-11-07T19:16:48Z")
> [@Xema](#): > > I think the notion is to measure the difference in gravitation over a smallish distance. This difference will be larger near a significant mass than when far away from one. No doubt. But how do you measure it? ---
**Author:** ![Mosier](https://avatars.discourse-cdn.com/v4/letter/m/7ea924/32.png) [@Mosier](https://boards.straightdope.com/u/Mosier)\ **Post date:** [November 7, 2009, 7:39pm UTC](https://boards.straightdope.com/t/help-my-coworker-understand-gravity-in-space/516581/65 "2009-11-07T19:39:14Z")
> [@Chessic\_Sense](#): > > It all started with astronauts on Fox News, being interviewed from the Space Station. A coworkers says to me “Wouldn’t it be cool to feel weightless like that?” I told him that, in fact, he _has_ felt that way anytime he’s been on a steep rollercoaster, a quickly descending plane, a falling elevator, trampoline, or swimming pool. He refused to believe this. > > His reasoning is that you get a knot in your stomach because you’re falling. And the astronauts just float around. I tried explaining that the astronauts are falling too, but he swears it’s not the same thing. He keeps saying that there’s no gravity in space. I ask what keeps the moon in orbit, and he says it’s because the moon is big (which I know to be irrelevant). He’s said that on the space station, he (a 270lb football player) would weigh nothing while I keep trying to explain that he’ll weigh roughly 250 lbs. And lastly, he says that there is gravity outside the station, but not inside it. > > It doesn’t help that other, worsely informed coworkers keep passing by to drop stupid-bombs by saying things like “If a plane flies really high, the passengers will be weightless” , “There are Lagrange points where there isn’t any gravity”, “The space station is outside the gravitational field of the earth” and “You can’t calculate gravity, you can only see it’s effects…like the wind.” > > These people have no understanding of force, velocity, acceleration, or a bunch of other Physics 101 concepts. So help me explain the following: > > 1. You’re not weightless on the space station. > 2. It’s exactly like being in a plummeting elevator. > 3. There’s gravity in space, inside the atmosphere, outside the atmosphere, in the space station, and outside the space station. > > Please help, before I lose my mind. The reason it’s NOT exactly like a falling elevator is because even though you’re in freefall on the space station, you’re not ACCELERATING. The knot you get in your stomach on a roller coaster is from acceleration, not from “falling”. ---
**Author:** ![Ponderoid](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/ponderoid/32/19221_2.png) [@Ponderoid](https://boards.straightdope.com/u/Ponderoid)\ **Post date:** [November 7, 2009, 8:09pm UTC](https://boards.straightdope.com/t/help-my-coworker-understand-gravity-in-space/516581/66 "2009-11-07T20:09:32Z")
> [@CookingWithGas](#): > > No doubt. But how do you measure it? [Tidal forces](http://en.wikipedia.org/wiki/Tidal_force) manifest as an apparent “stretching” from the POV of your spacecraft, towards and away from nearby masses. A simple experiment you could do inside your spacecraft is to first, turn off your air circulation system, and then scatter a bunch of small objects around your spacecraft, leaving them motionless relative to the ship to start off with. Then wait. While waiting, put on an oxygen mask or spacesuit, or keep moving, otherwise you’ll suffocate due to the globe of exhaled CO[sub]2[/sub] around your head. 🙂 After a while, you’ll notice the objects on one side of the spacecraft will drift preferentially toward that side, and objects on the other side will go the opposite way. The nearby major mass is in one of those 2 directions. You won’t be able to tell which is which without being able to see outside. This tiny acceleration is why, strictly speaking, “zero-g” isn’t the right term anymore for what astronauts experience in orbit, and properly researched publications say “[microgravity](http://en.wikipedia.org/wiki/Microgravity)” these days. If you’ve got a couple super-accurate atomic clocks, you can use [gravitational time dilation](http://en.wikipedia.org/wiki/Gravitational_time_dilation) from general relativity to tell you which way is which. The clock closer to the large mass will run slower. \*\*\* Ponder ---
**Author:** ![Kevbo](https://avatars.discourse-cdn.com/v4/letter/k/e47774/32.png) [@Kevbo](https://boards.straightdope.com/u/Kevbo)\ **Post date:** [November 7, 2009, 8:34pm UTC](https://boards.straightdope.com/t/help-my-coworker-understand-gravity-in-space/516581/67 "2009-11-07T20:34:42Z")
> [@CookingWithGas](#): > > . Such weightlessness is often simluated for training purposes in airplanes that dive, like NASA’s [Vomit Comet](http://www.carthage.edu/articles/nasa2.cfm). nit: You may well know better, but it is a common misunderstanding that something must be _moving_ downward in order to be in free fall. In fact it needs to be _accelerating_ (at 1 g) downward.\* Indeed for an orbiting body, all motion is sideways, with continuous downward acceleration. The downward acceleration condition can also be satisfied along with motion in other diredtions, including upward. Yes, upward. When you throw a ball upward into the air, it is in free fall from the moment it leaves your hand, while it is still coasting upward…not beginning at the top of it’s arc. Indeed, if you threw it at ,or greater than, escape velocity, it would stay in free fall with upward motion indefinitely. Which gets to why I quoted the bit above: If the pilot just pushed into a dive from level flight, he would need to start at or above stall speed, and would over-speed within just a few seconds. In order to maximize free fall time, the training aircraft starts it’s “fall” at high speed with a hard pull _upward_. Weightlessness begins while the aircraft is still traveling upward. The top of the flight path can also be taken at well below normal “stall speed”\*\* because the wings do not need to support any weight. Only enough speed to keep the controls effective needs to be maintained. I have actually gotten the ASI in a glider on the lower peg without stalling in this way. footnotes: \*I define “downward” as toward the center of the earth for the purposes of this post. \*\*Aerodynamic stall happens due to excessive angle of attack, NOT due to insufficient air speed..A distinction with significance even many pilots fail to grasp. “Stall speed” scales upward and downward with the square root of the G-load. The mark on the ASI is valid oly at 1g. ---
**Author:** ![Kevbo](https://avatars.discourse-cdn.com/v4/letter/k/e47774/32.png) [@Kevbo](https://boards.straightdope.com/u/Kevbo)\ **Post date:** [November 7, 2009, 9:05pm UTC](https://boards.straightdope.com/t/help-my-coworker-understand-gravity-in-space/516581/68 "2009-11-07T21:05:51Z")
> [@Mosier](#): > > The reason it’s NOT exactly like a falling elevator is because even though you’re in freefall on the space station, you’re not ACCELERATING. Nope. The space station IS constantly accelerating downward at 1(local) g, just like the failed elevator. If this were not true, it (Space station) would follow a straight path instead of an approximately circular orbit. ---
**Author:** ![CookingWithGas](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/cookingwithgas/32/485_2.png) [@CookingWithGas](https://boards.straightdope.com/u/CookingWithGas)\ **Post date:** [November 7, 2009, 9:29pm UTC](https://boards.straightdope.com/t/help-my-coworker-understand-gravity-in-space/516581/69 "2009-11-07T21:29:44Z")
> [@Kevbo](#): > > Nope. The space station IS constantly accelerating downward at 1(local) g, just like the failed elevator. If this were not true, it (Space station) would follow a straight path instead of an approximately circular orbit. One thing that may confusing to the layperson is that the speed of the space station remains (more or less) constant, while the direction of the velocity vector is constantly changing. To explain this change in physics we show an acceleration vector pointed to the center of the orbit (assuming the simplified case of a circular orbit). [This diagram](http://physics-help.info/physicsguide/mechanics/rotational_kinematics.shtml) is a little busy but this acceleration is shown as \_ _a[sub]n_[/sub] ---
**Author:** ![chorpler](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chorpler/32/2888_2.png) [@chorpler](https://boards.straightdope.com/u/chorpler)\ **Post date:** [November 7, 2009, 9:53pm UTC](https://boards.straightdope.com/t/help-my-coworker-understand-gravity-in-space/516581/70 "2009-11-07T21:53:35Z")
> [@Mosier](#): > > The reason it’s NOT exactly like a falling elevator is because even though you’re in freefall on the space station, you’re not ACCELERATING. The knot you get in your stomach on a roller coaster is from acceleration, not from “falling”. A couple of people have made this claim now (that you don’t feel the knot in your stomach in free-fall on the space station), but other people have seemed to refute it. What’s the truth? ---
**Author:** ![Whack-a-Mole](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/whack-a-mole/32/141_2.png) [@Whack-a-Mole](https://boards.straightdope.com/u/Whack-a-Mole)\ **Post date:** [November 7, 2009, 10:38pm UTC](https://boards.straightdope.com/t/help-my-coworker-understand-gravity-in-space/516581/71 "2009-11-07T22:38:51Z")
> [@Nancarrow](#): > > Jesus Tittyfucking Christ. > > Yes, they do use the term weightless. Ill-advisedly, but whatchagonna do. > > Now look at the diagrams. > > And the big equation that says > > W = m g > > What definition of weight is used in the diagrams and equation? The one I’ve been banging on about. The useful one. The one that helps clear confusion on this subject. Most of the time. > > Fuck me, they only use the term “weightless” once AND THEY PUT IT IN SCARE QUOTES. > > Am I ever again going to look at a bowl of one of my favourite soups in the same way? > > And **Yumblie** , please do follow **CookingWithGas** ’s advice. And check out the links I referred to, particularly the NASA one. > > Now I damn well AM done here. Not trying to be a pain, genuinely curious. Let’s say you are on the space station. You are born there in fact. There are no windows in the space station, for all intents and purposes to you it is the whole universe. Is there _any_ experiment, atomic clocks or whatever, you can do there that would tell you that you are, in fact, in a gravitational well of a planet (or just in a gravity well)? Anything that would suggest you actually do have weight which is merely being counteracted by motion? Remember the Equivalence Principle, acceleration is indistinguishable from gravity in its effects. So, while you are in a gravity well it is being counteracted by your acceleration (circle) around the planet. Also remember that no reference frame is favored as the “correct” reference frame. Each one is as valid as the next. So you cannot say the space station person is “wrong” because he lacks some info. In his universe he is right…truly and actually right. It seems to me the hang up is saying we _know_ the astronauts are actually in orbit and in a gravity well so while they will measure zero weight that is really just a trick. From our perspective here on Earth we know they weigh something. It is conflating reference frames. If however you tell me that the astronaut could indeed do experiments on the space station that would indicate he is in a gravity well, that his reference frame really is not an actual reference frame, then I am with you and a lot of my confusion will be cleared. ---
**Author:** ![Chessic\_Sense](https://avatars.discourse-cdn.com/v4/letter/c/7c8e57/32.png) [@Chessic\_Sense](https://boards.straightdope.com/u/Chessic_Sense)\ **Post date:** [November 8, 2009, 12:27am UTC](https://boards.straightdope.com/t/help-my-coworker-understand-gravity-in-space/516581/72 "2009-11-08T00:27:16Z")
> [@pancakes3](#): > > what then were you talking about when you said that the artificial gravity generated convinced your friend of the existence of gravity in the space station? it has absolutely nothing to do with gravity. he is no closer in understanding the effects of gravity on a space station than he was a day ago - possibly more confused. > > also, what calculations were you doing involving doctors and babies? a doctor holding a baby out vs holding a baby close to the vest? I was saying that the artificial gravity stories worked _againt_ his understanding of the subject. But then he read something else (I don’t know what) and changed his mind. The calculations were m/r^2. I took the mass of Jupiter and it’s closest and farthest distances from Earth and came up with a number. Then I took a 100kg doctor and put him half a meter from the baby, and got another number. A close Jupiter is gravitationally equivalent to 10 doctors, while a far one is roughly 4.5 doctors. It was just a silly little anti-astrology back-of-the-envelope calculation. ---
**Author:** ![Frylock](https://avatars.discourse-cdn.com/v4/letter/f/ce7236/32.png) [@Frylock](https://boards.straightdope.com/u/Frylock)\ **Post date:** [November 8, 2009, 1:12am UTC](https://boards.straightdope.com/t/help-my-coworker-understand-gravity-in-space/516581/73 "2009-11-08T01:12:51Z")
> [@gazpacho](#): > > I read those links and they are talking about how your are weightless in free fall. Are you being serious? The Nasa link puts the word “weightless” in scare quotes, and moreover, begins with the following: > [@](#): > > An object that falls through a vacuum is subjected to only one external force, the gravitational force, expressed as the weight of the object. The weight equation defines the weight W to be equal to the mass of the object m times the gravitational acceleration g: > > W = m \* g ---
**Author:** ![Stealth\_Potato](https://avatars.discourse-cdn.com/v4/letter/s/d78d45/32.png) [@Stealth\_Potato](https://boards.straightdope.com/u/Stealth_Potato)\ **Post date:** [November 8, 2009, 1:21am UTC](https://boards.straightdope.com/t/help-my-coworker-understand-gravity-in-space/516581/74 "2009-11-08T01:21:36Z")
> [@chorpler](#): > > A couple of people have made this claim now (that you don’t feel the knot in your stomach in free-fall on the space station), but other people have seemed to refute it. What’s the truth? The truth is that you are definitely accelerating when you are in orbit, just the same as you are accelerating when you are in a falling elevator. If you were not accelerating, you could not possibly maintain orbit: you would go flying off into deep space. It is the force of gravity on you that pulls you into an elliptical orbit around the planet. The difference between freefall and standing on the ground is simply that in the latter case, there is a force holding your body stationary with respect to the ground. This force is transmitted through your body pretty much throughout your life, and as a result your body is accustomed to having all of its inner bits pressing against each other in the downward direction; the direction of that force is what you use to sense your orientation. Without something pressing against you in opposition to the gravitational force, your various organs (including the ones you use for balance and orientation) are free-floating, and this feels really weird. You feel it to a small extent when you first begin a descent in an elevator, and to a much greater extent on some amusement park rides. Astronauts in orbit would have to learn to deal with it for long periods of time. I’ve never been myself, but I’m guessing it’s something you get used to. ---
**Author:** ![Francis\_Vaughan](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/francis_vaughan/32/3093_2.png) [@Francis\_Vaughan](https://boards.straightdope.com/u/Francis_Vaughan)\ **Post date:** [November 8, 2009, 4:09am UTC](https://boards.straightdope.com/t/help-my-coworker-understand-gravity-in-space/516581/75 "2009-11-08T04:09:21Z")
> [@Whack-a-Mole](#): > > If however you tell me that the astronaut could indeed do experiments on the space station that would indicate he is in a gravity well, that his reference frame really is not an actual reference frame, then I am with you and a lot of my confusion will be cleared. The trick is that the space station is an extended solid object. Its orbit and movement is defined by its centre of mass. But if you move away from the centre of mass you are shifting reference frames. The gravity isn’t constant across the extent of the space station, and if the station was not solid it would slowly stretch out. Given enough time it would spread itself right around the earth. A nice example of this being Saturn’s rings. This is what you measure to find the nature of your outside world. You look for the anomoly that says that the extended space station isn’t a single reference frame, although it is a solid physical object. The above described mechanisms allow you to find this. They allow you to look for the very slight differences in gravitation field across the station. ---
**Author:** ![CC](https://avatars.discourse-cdn.com/v4/letter/c/f1d935/32.png) [@CC](https://boards.straightdope.com/u/CC)\ **Post date:** [November 8, 2009, 4:11am UTC](https://boards.straightdope.com/t/help-my-coworker-understand-gravity-in-space/516581/76 "2009-11-08T04:11:27Z")
Here’s one source of confusion in the above posts: the word, acceleration refers to any change of velocity. Consequently, when the direction of travel changes, as when the space station moves in a circular direction around the earth, it is accelerating. ---
**Author:** ![Punoqllads](https://avatars.discourse-cdn.com/v4/letter/p/d2c977/32.png) [@Punoqllads](https://boards.straightdope.com/u/Punoqllads)\ **Post date:** [November 8, 2009, 4:54am UTC](https://boards.straightdope.com/t/help-my-coworker-understand-gravity-in-space/516581/77 "2009-11-08T04:54:44Z")
> [@chorpler](#): > > A couple of people have made this claim now (that you don’t feel the knot in your stomach in free-fall on the space station), but other people have seemed to refute it. What’s the truth? The [centripetal force](http://en.wikipedia.org/wiki/Centripetal_force) on an object moving in a circle around a central point is m \* v[sup]2[/sup]/r, where m is the mass, v the velocity of the object, and r the distance from the centroid of the object to the central point. Since F = m \* a, we can say that a = v[sup]2[/sup]/r. The object is accelerating. ---
**Author:** ![jackdavinci](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/jackdavinci/32/3259_2.png) [@jackdavinci](https://boards.straightdope.com/u/jackdavinci)\ **Post date:** [November 8, 2009, 8:24am UTC](https://boards.straightdope.com/t/help-my-coworker-understand-gravity-in-space/516581/78 "2009-11-08T08:24:28Z")
While gravitationally speaking, you are right in that the two things are equivalent, you are also wrong in that obviously they are very different experientially. Yes, in both cases there is weightlessness at some point, but the feeling of rapidly changing acceleration, part of which is a net of 0 while seeing changing scenery and the sesation of plummeting towards the ground, is going to ‘feel’ a lot different than experiencing just constant weightlessness within a confined area with no visual hints of acceleration. Being in a pool underwater probably ‘feels’ more like astronauts weightlessness than rollercoasters and elevators. Ideally of course would be a ride on the parabola plane. ---
**Author:** ![Francis\_Vaughan](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/francis_vaughan/32/3093_2.png) [@Francis\_Vaughan](https://boards.straightdope.com/u/Francis_Vaughan)\ **Post date:** [November 8, 2009, 8:33am UTC](https://boards.straightdope.com/t/help-my-coworker-understand-gravity-in-space/516581/79 "2009-11-08T08:33:08Z")
> [@jackdavinci](#): > > Being in a pool underwater probably ‘feels’ more like astronauts weightlessness than rollercoasters and elevators. Ideally of course would be a ride on the parabola plane. Except for visceral bits. Inner ear and guts floating about. So it is hard to approximate. But the point about the visual clues is very important, and the sudden changes. So, that gets us back to the vomit comet as the only really good way. A worthwhile point. The scenes in the movie of Apollo 13 that depicted weightlessness were shot in the (or at least a) vomit comet. Took quite a while, but movies nowadays are pretty fast edited anyway, so building the movie up to look realistic from a heap of few second shots was always possible. One imagines the amount of shooting time between start of zero g, setting up the shot and rolling was pretty tight. ---
**Author:** ![Nancarrow](https://avatars.discourse-cdn.com/v4/letter/n/c37758/32.png) [@Nancarrow](https://boards.straightdope.com/u/Nancarrow)\ **Post date:** [November 8, 2009, 12:16pm UTC](https://boards.straightdope.com/t/help-my-coworker-understand-gravity-in-space/516581/80 "2009-11-08T12:16:45Z")
> [@Whack-a-Mole](#): > > Remember the Equivalence Principle, acceleration is indistinguishable from gravity in its effects. So, while you are in a gravity well it is being counteracted by your acceleration (circle) around the planet. Well it’s been a while since I studied GR and I can’t claim to have understood more than the basics, but AFAIK the equivalence principle only holds _either_ for a completely uniform gravitational field, _or_ if the field is non-uniform, it is only true in the limit as you consider smaller and smaller pieces of space. So for example, if you’re in a falling elevator, say a couple of hundred feet drop near the Earth’s surface, you can pretty much use the equivalence principle. The grav field is about as uniform as you can hope for (or you can just as well say that, though it is non-uniform, the space in the elevator is so small that you may as well say it’s uniform). So your environment would indeed be indistinguishable, to an excellent first approximation, from being in the inky depths of intergalactic space. At least until you went splat. But now suppose you’re in a damn big elevator and it’s quite some distance from the earth and it’s falling towards it. So we’re talking about a crashing spaceship basically. If you now place a couple of ping pong balls in the air, say several hundred metres apart (within the spaceship), and watch their behaviour, _they will move closer together_. This is because the weight vectors on them are not precisely parallel, they both go towards the centre of the Earth. So while they both go ‘down’, down is in slightly different directions for each ball. > [@](#): > > If however you tell me that the astronaut could indeed do experiments on the space station that would indicate he is in a gravity well, that his reference frame really is not an actual reference frame, then I am with you and a lot of my confusion will be cleared. So returning to your question, if they were truly not in a planetary grav field, you could suspend the ping pong balls in the air (or let’s make it an evacuated chamber!) and they’d stay motionless relative to each other forever. But on the space station they would ever so slowly move relative to each other due to the very slight non-uniformity of the gravitational field in their region of space. In principle you could detect this, given sufficient patience, experimental design care, and equipment sensitivity. > [@](#): > > Also remember that no reference frame is favored as the “correct” reference frame. Each one is as valid as the next. I’m not too sure about this. Both Newton and Einstein pick out a special class of ‘priviledged’ reference frames called _inertial_ reference frames, though annoyingly (but very importantly to the development of each theory) the definitions are different. For Newton, inertial reference frames are non-accelerating\*, whereas for Einstein, they’re ones in free-fall in a gravitational field (_any_ grav field, not just a uniform one). I suppose it’s semantics to ask whether the distinction between inertial and non-inertial is also one of ‘correctness’, but just sticking to Newton for the mo, it’s certainly a distinction between ‘simple’ and ‘complicated’ in a specific sense. If you are in an inertial reference frame, you can apply Newton’s Laws without further ado. But suppose you’re in a non-inertial reference frame like a spinning space station. The spinning space station _apparently_ provides a ‘force’ pulling you towards the ‘ground’ (i.e. the rim of the space station) (and you can do weird things like throw a ball straight up in the air and have it curve to the side on the way down). But where is this force coming from? Newton’s laws do not say. It can’t be gravity, the space station has far too little mass, and anyway, the grav force from the part beneath your feet would be counteracted by the force from the rest of the station at your sides and above you. So what’s going on? You have two choices. Firstly you can simply say that Newton’s laws no longer hold in your ref frame, you can’t use them to calculate anything, bad luck. But there’s a second better way of doing it. Postulate that there is a force directed radially outward from the centre of the station, which increases with distance from that centre - call it [‘running away from the centre’](http://en.wikipedia.org/wiki/Centrifugal_force) force if you like, but use Latin to sound more professional ;). Then, provided you remember to include that force on all free-body diagrams for anything you throw up in the air, you can use Newton’s laws. That “centrifugal” force is called a ‘pseudo-’ force or ‘fictitious’ force. It isn’t _real_, in the sense that it does not arise from the interaction between the body it’s acting on, and any other. It is actually just an artifact of you being in a non-inertial (i.e. accelerated) reference frame. Considerations of the distinction between ‘inertial’ and ‘non-inertial’ frames in Newtonian mech, combined with thinking about uniform grav fields, led Einstein to formulate the equivalence principle. This led to GR, in which gravity is no longer considered a ‘force’ in the Newtonian sense, but a distortion of space time. So a freely falling object, for which the only ‘force’ acting is gravitational, Einstein says there are NO forces acting on it. Hence Newton’s inertial frames morph into Einstein’s inertial frames. Hope that helps! Of course all this stuff would really do no good for **Chessic Sense** ’s co-worker, but apparently they have already seen the light, and they’re not reading this anyway! 🙂 \*relative to what? Well it’s a kind of bootstrap approach. Pick any reference frame in which Newton’s laws hold. Define it as inertial. Then any reference frame that is non-accerating relative to that one, can be shown to also be inertial. [Previous page](https://boards.straightdope.com/t/help-my-coworker-understand-gravity-in-space/516581.md?page=3) [Next page](https://boards.straightdope.com/t/help-my-coworker-understand-gravity-in-space/516581.md?page=5)