# Space station rotation **URL:** \ **Category:** Cecil's Columns/Staff Reports\ **Created:** [January 10, 2003, 2:10am UTC](https://boards.straightdope.com/t/space-station-rotation/147657 "2003-01-10T02:10:43Z")\ **Posts on this page:** 10\ **Page:** 3
**Author:** ![erislover](https://avatars.discourse-cdn.com/v4/letter/e/71e660/32.png) [@erislover](https://boards.straightdope.com/u/erislover)\ **Post date:** [January 26, 2003, 8:07am UTC](https://boards.straightdope.com/t/space-station-rotation/147657/41 "2003-01-26T08:07:18Z")
But, in fact, why don’t I set the record straight here. I don’t care if you want to talk all day about a non-intertial frame. I certainly wasn’t, so if you were, then I’m sorry for thinking you’d stay on track. > [@](#): > > Your original statement was that there was an acceleration upward, which is manifestly false: the only acceleration is downward. I’m sorry, but that’s bullshit any way you slice it. There is a net accelleration toward the center, of this we seem to agree. **There can be no net anything in one direction unless there is another, lesser something in another direction.** You want to consider only forces, fine, be my guest—the math is the same, and we will describe the same circular motion. When I study motion, I tend to look at and do the math with accelleration, which is the result of a force acting on a mass. We are moving in a (rough) circle. The accelleration due to gravity is too much to do this with the radius and velocity in question. There must be some component vector that counters this. You want to work only with forces? Fine. Mathematically equivalent since F=m\*a. I’m going to work with accelleration. If you disagree, show me the math, and show me why I am wrong there. Please. Otherwise, your childish ranting will be wasted on deaf ears. ---
**Author:** ![C\_K\_Dexter\_Haven](https://avatars.discourse-cdn.com/v4/letter/c/b2d939/32.png) [@C\_K\_Dexter\_Haven](https://boards.straightdope.com/u/C_K_Dexter_Haven)\ **Post date:** [January 26, 2003, 3:52pm UTC](https://boards.straightdope.com/t/space-station-rotation/147657/42 "2003-01-26T15:52:52Z")
**Nametag** says: \<\< I will concede, though, that you are less ignorant of physics than you are of English. \>\> **erislover** responds: \<\< … your childish ranting will be wasted on deaf ears. \>\> Administrator claps his hands, VERY LOUD, to attract attention. OK, children, enough of that. Name calling is NOT permitted in this forum. If you want to do name-calling, go to the forum called the BBQ Pit. This forum is limited to polite discussion or debate, and taunts such as the above are not permitted. Consider this a formal warning. Play nice, or play elsewhere. ---
**Author:** ![erislover](https://avatars.discourse-cdn.com/v4/letter/e/71e660/32.png) [@erislover](https://boards.straightdope.com/u/erislover)\ **Post date:** [January 26, 2003, 6:13pm UTC](https://boards.straightdope.com/t/space-station-rotation/147657/43 "2003-01-26T18:13:34Z")
Aye. ---
**Author:** ![Devil\_s\_Advocate](https://avatars.discourse-cdn.com/v4/letter/d/3d9bf3/32.png) [@Devil\_s\_Advocate](https://boards.straightdope.com/u/Devil_s_Advocate)\ **Post date:** [January 27, 2003, 11:55pm UTC](https://boards.straightdope.com/t/space-station-rotation/147657/44 "2003-01-27T23:55:59Z")
OK, hopefully I can clear up some of the confusion here. **erislover** , it’s specifically _net_ force on an object that is equal to its acceleration times its mass. Acceleration is the rate of change of velocity; an object has only one acceleration. Of course, for each force acting on the object, there is an acceleration that _would_ result if that force alone were acting on the object, but to casually speak of one of these as a real “acceleration” is a misuse of the term. The _only_ acceleration is “net” acceleration. There are, however, forces other than the net force, which is really just the sum of the actual forces acting on the object. And that’s why we should properly talk about forces instead of accelerations on an object in a discussion like this. You’re right about the mathematical equivalence of the descriptions (for an object of constant mass), but, well, it’s a semantic thing. Now then, **Nametag** , you said > [@](#): > > You do see, don’t you, that a person standing on the Earth’s surface is stationary in such a frame of reference? That a stationary person, by definition, is not accelerating? That there is therefore no net force? Ahem. When a ball, thrown upwards, reaches its maximum height, it has a velocity of zero, but still experiences a net force and an acceleration. A stationary object has no _velocity_ by definition, but it doesn’t follow that it’s not accelerating; a nonmoving object can have an acceleration. I’ll grant you, you could (re)define “stationary” so that it only refers to something that has no velocity _for more than just one instant_, which is clearly what you’re talking about, and which would be reasonable enough. But that would be weaseling, in my view. But even worse, you said > [@](#): > > No, erislover, we do NOT have an acceleration up. We have an acceleration DOWN – more accurately, we have an acceleration toward the center of the Earth, caused by gravity. If we did not have such an acceleration, we would be flying away from the Earth. To say that someone standing on the Earth is accelerating downward is almost as bad as saying that (s)he is accelerating upward. Technically you’re correct; there’s a net downward centripital force that keeps us moving in a circle around the Earth, which is spinning. However, in this case, I think that the intuitive view is to use the reference frame of the person standing on the Earth. (Yes, I know that that contradicts what I said before about the inertial frame being assumed “correct” by convention. Look, it’s all a matter of context. No, really, it is! Shut up.) From that view, you seem to be misusing the term “acceleration” in the same way as **erislover** later did, and I think that this may have contributed to his/her confusion about the use of that term in the first place. Besides, if you really want to look at things non-inertially, you’ve also got to consider that not only is the Earth spinning, it’s going 'round the sun, and the solar system itself is orbiting around the center of the galaxy, etc.! Oh, and for completeness, I guess I should add that I’m assuming that we _are_ talking about someone standing on the ground, not in free fall. That _was_ what we were discussing, right? So, given the right assumptions, I think everything you said was technically correct. But since you didn’t explain those assumptions, you stated a few things in a fashion that some people may have found misleading. So ease up, OK? ---
**Author:** ![erislover](https://avatars.discourse-cdn.com/v4/letter/e/71e660/32.png) [@erislover](https://boards.straightdope.com/u/erislover)\ **Post date:** [January 28, 2003, 12:32am UTC](https://boards.straightdope.com/t/space-station-rotation/147657/45 "2003-01-28T00:32:27Z")
> [@](#): > > …an object has only one acceleration. Of course, for each force acting on the object, there is an acceleration that would result if that force alone were acting on the object, but to casually speak of one of these as a real “acceleration” is a misuse of the term. Well, I’ve never heard this. I can live with it, no problem. 🙂 I think in terms of the math, and the math often uses component vectors. For the record, say you were studying a rocket propelled sideways along the earth and you knew the force the rocket put out. Why would the one component vector of accelleration be less real than the net accelleration from the addition of the vectors? Is this a stupid question? ---
**Author:** ![Devil\_s\_Advocate](https://avatars.discourse-cdn.com/v4/letter/d/3d9bf3/32.png) [@Devil\_s\_Advocate](https://boards.straightdope.com/u/Devil_s_Advocate)\ **Post date:** [January 28, 2003, 1:14am UTC](https://boards.straightdope.com/t/space-station-rotation/147657/46 "2003-01-28T01:14:56Z")
Oh, I just noticed that **erislover** said the following: ## "quote: ## There IS, however, another force acting on most objects, the force exerted by the Earth itself, equal and opposite to the force that we exert on it. Sorry, no. If this were true we wouldn’t travel in a circle, we’d fly right off the earth. Why don’t you check my math?" That’s exactly correct, if you’re using the solar system as your frame of reference; gravity is somewhat stronger than the normal force (the force that keeps solid objects from moving through each other), and this centripital net force keeps us moving around the Earth. Only from the non-inertial frame of someone standing on the Earth are the two forces equal and opposite. ## "quote: ## When viewed in an INERTIAL frame of reference that is stationary with respect to the Earth’s center of rotation, this force is slightly less than, but still opposite to, the force of gravity, and there is a net CENTRIPETAL acceleration – that is, DOWN. Yes, a net cetripetal accelleration. Can you tell me why you use the word “net” without proving my point?" Indeed. Unless you are (mis)using the term “acceleration” as **erislover** did, “net acceleration” is redundant; in fact, it implies that there are “component” accelerations. In the future, **Nametag** , try to be more consistent in the way you talk about something. Even if there are two equally correct but conflicting ways of describing something, switching between the two without notice can lead to (apparent) contradiction. ---
**Author:** ![Devil\_s\_Advocate](https://avatars.discourse-cdn.com/v4/letter/d/3d9bf3/32.png) [@Devil\_s\_Advocate](https://boards.straightdope.com/u/Devil_s_Advocate)\ **Post date:** [January 28, 2003, 1:22am UTC](https://boards.straightdope.com/t/space-station-rotation/147657/47 "2003-01-28T01:22:49Z")
> [@](#): > > \*Originally posted by erislover \* > \*\*Well, I’ve never heard this. I can live with it, no problem. 🙂 I think in terms of the math, and the math often uses component vectors. > > For the record, say you were studying a rocket propelled sideways along the earth and you knew the force the rocket put out. Why would the one component vector of accelleration be less real than the net accelleration from the addition of the vectors? Is this a stupid question? \*\* It’s not that the component vectors aren’t “real”, but a component of the object’s acceleration isn’t “an acceleration of the object”, only the “net” acceleration is. Or to put it another way, “net acceleration” is redundant. At least I _hope_ I’ve got that right. I mean, I’m gonna look pretty stupid going on about it like I did if I’m wrong. 😃 ---
**Author:** ![Irishman](https://avatars.discourse-cdn.com/v4/letter/i/b487fb/32.png) [@Irishman](https://boards.straightdope.com/u/Irishman)\ **Post date:** [January 28, 2003, 8:47am UTC](https://boards.straightdope.com/t/space-station-rotation/147657/48 "2003-01-28T08:47:28Z")
In all my physics and dynamics and engineering classes, I never encountered anyone making a big deal about it. We talked about component accelerations all the time. When breaking down forces by vectors, we also broke down accelerations by vectors. An object is seeing independent accelerations, it’s just they sum vectorially. Same difference. ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [January 30, 2003, 5:05am UTC](https://boards.straightdope.com/t/space-station-rotation/147657/49 "2003-01-30T05:05:48Z")
> [@](#): > > * * * > > ## There IS, however, another force acting on most objects, the force exerted by the Earth itself, equal and opposite to the force that we exert on it. > > Sorry, no. If this were true we wouldn’t travel in a circle, we’d fly right off the earth. Why don’t you check my math?" **erislover** is correct in this. The force exerted by the Earth on any object is exactly equal in magnitude (and opposite in direction) to the force that object exerts on the Earth. This is Newton’s Third Law, and it is true regardless of the relative motion of the Earth and the other object, or of the reference frame used. The total force of the Earth on an object has two parts: A gravitational part and a contact-force part. If we neglect the rotation of the Earth, and look at an object resting on the surface, it is also true that these two parts to the force are equal in magnitude and opposite in direction. This is a special case of Newton’s Second Law ( **F** [sub]net[/sub]=m **a** , where **a** = 0). Therefore, the Earth exerts zero net force on such an object. If we do not neglect the rotation of the Earth, and consider an object resting on that surface, then the gravitational force must be slightly larger than the contact force, for a small net force downwards, which is keeping the object moving in a circle about the center of the Earth. I think that’s what you’re referring to. ---
**Author:** ![Nametag](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/nametag/32/406_2.png) [@Nametag](https://boards.straightdope.com/u/Nametag)\ **Post date:** [January 30, 2003, 6:22pm UTC](https://boards.straightdope.com/t/space-station-rotation/147657/50 "2003-01-30T18:22:56Z")
**Chronos** , I believe that’s my point you’re agreeing with. **Devil’s Advocate** , of course “net acceleration” is redundant. You have to do that when people don’t get it the first time. Moreover, a broad definition of "stationary as “not moving” is not weaseling; that term is reserved for narrow definitions such as “having reached an instantaneous velocity of zero during the ballistic course of flight of an obviously moving object.” Obviously, acceleration can be separated into component vectors; had the words “vector” or “component” been used in the first place, there would be no argument. But both Erislover and I were addressing **CurtC** ’s erron:eous contention that > [@](#): > > …if you’re talking about a non-rotating frame of reference, why would you state that “you’ve got a constant acceleration, towards the center of the space station”? This statement definitely tells me that the person is trying to convey the perspective of the dude on the space station. If you were talking about the non-rotating reference, wouldn’t you say “the floor must exert a constant acceleration…”? > > Saying that you’ve got a constant acceleration towards the center is like saying that here on Earth we have a constant acceleration _up_. It’s clear that CurtC’s post is referring to the actual acceleration experienced by an object, not some smaller component of it. I didn’t immediately catch on that erislover’s one-line quip was about the acceleration component generated by the Earth’s contact force. It’s now been made abundantly clear that this is the case, so erislover’s reply was not wrong, but it was sloppy and irrelevant. [Previous page](https://boards.straightdope.com/t/space-station-rotation/147657.md?page=2)