Path: senator-bedfellow.mit.edu!bloom-beacon.mit.edu!newsfeed.stanford.edu!ediacara.org!there.is.no.cabal From: "sds" Newsgroups: talk.origins,talk.answers Subject: Re: Why we feel the *EFFECT OF WEIGHT*. Date: 28 Jan 2002 00:14:29 -0500 Organization: MindSpring Enterprises Lines: 209 Approved: robomod@ediacara.org Message-ID: References: <6f995fdf.0201190919.75993476@posting.google.com> <6f995fdf.0201201834.155afd52@posting.google.com> <3c4ebe50.780344926@news.cc.umanitoba.ca> <3c503319.875777201@news.cc.umanitoba.ca> <3c532525.4864585@news.cc.umanitoba.ca> NNTP-Posting-Host: darwin.ediacara.org X-Server-Date: 28 Jan 2002 05:14:12 GMT X-Priority: 3 X-MSMail-Priority: Normal X-Newsreader: Microsoft Outlook Express 5.50.4133.2400 X-MimeOLE: Produced By Microsoft MimeOLE V5.50.4133.2400 Xref: senator-bedfellow.mit.edu talk.origins:894471 talk.answers:5745 "Don Cates" wrote in message news:3c532525.4864585@news.cc.umanitoba.ca... > On 24 Jan 2002 13:57:06 -0500, "sds" wrote: > > > > >"Don Cates" wrote in message > >news:3c503319.875777201@news.cc.umanitoba.ca... > >> On 23 Jan 2002 23:21:06 -0500, "sds" wrote: > >> > >> > > >> >"Don Cates" wrote in message > >> >news:3c4ebe50.780344926@news.cc.umanitoba.ca... > >> >> On 23 Jan 2002 01:06:01 -0500, "sds" wrote: > >> >> > >> >> >Leave it to you, Jeff, to actually think about this and reply. > >I'm > >> >> >sure your comments are correct, but since I happen to actually > >know > >> >> >something about this non-t.o. subject, I'll try to argue with > >you a > >> >> >little :) > >> >> > > >> >> >> >Does a simple spring/mass accelerometer such as is used in > >> >> >> >old-fashioned basic guidance systems measure gravity? > >> >> >> > > >> >> >> > >> >> >> Trying to analyze accelerometer data to reconstruct the motion > >of > >> >an > >> >> >> object is amazingly confusing until you figure this one out... > >> >> > > >> >> >In fact, in general, one cannot reconstruct the motion of an > >object > >> >> >based solely on it's accelerometer trace. Two additional pieces > >of > >> >> >the puzzle are required - the orientation of the instrument, > >> >> >obviously, and the time history of the gravitational > >acceleration > >> >> >vector local to the instrument. > >> >> > >> >> And the initial conditions. > >> >> > >> >> > Yes, I'm talking large 3D motion. > >> >> >Obvious simplifications could be made for other applications. > >> >> > >> >> >> The trick > >> >> >> is to recognize that an accelerometer measures acceleration > >> >> >differences > >> >> >> between its housing and its test mass. That's all it does. > >> >> > >> >> I always thought of it as measuring the net force on the housing. > >> > > >> >I can't quite accept that either - I don't think. When on the > >table, > >> >the net force on the housing is zero > >> > >> Really? Then I take there would be no frictional forces to oppose > >the > >> housing sliding on the table. (I belive something called the normal > >> force is required for that) > > > >Yes, and if not for that normal force, the housing would then have a > >net nonzero force and would fall. > > I suspect we both know what is really happening. I just have an issue > with the teminology used. IMO gravity should *always* be viewed as an > acceleration. Associating a 'force' with gravity is fraught with > problems. It is not an intrinsic part of gravity but depends on the > mass of the object in the gravitational field. ISTM that any > independent measure of force on the housing would only see an upward > force on the base. Any downward 'force' can only be inferred from the > relative motion of the housing. > > >Look, I'm not trying to argue *this stuff* for sure. I'll stick to > >evolution for my arguments. > > > >If you would like me to explain something, I'm happy to. But I'm down > >right tired of all the confrontations from so many out there. > > > >You may indeed not understand this accelerometer thing. I don't know. > >If you don't, I certainly don't hold that against you. I would be > >happy to help anybody understand anything that I happen to be familiar > >with. > > Oh, I think I understand it well enough. Perhaps I did not explain my > point of view very well. > > >If I've misunderstood your "attitude", I'm sorry. > > I was trying to be 'light hearted' rather than 'confrontational'. > Looking back, I can see that it could quite reasonably be taken > otherwise. No opology necessary (except by me). Cates, I almost dropped my keyboard. You must not know my reputation on t.o. I can't think of more than a small handful of times that anybody has so much as hinted at an apology to me. Maybe twice. And none is needed here (but I sure appreciate the gesture!). Part of my problem is that I have too many people arguing with me in too many threads. If my attitude is negative because of a couple of other threads (which it is!), then I tend to forget just what's going on elsewhere, such as here. > >The housing is acted on by gravity and the normal force from the > >table. They balance to a net of zero which is why the object remains > >stationary. So, the net force on the housing is zero. > > Do you think it is at least *reasonable* to think of this as an upward > force giving an upward acceleration in a gravitational field providing > the equivalent of a downward acceleration? This makes the readings > from the accelerometer consistent in interpretation. And you seem to be actually asking *me* something! You clearly haven't followed me elsewhere. Good! Yes, it's more than reasonable, it's perfectly accurate. The forces on the housing - the measurements a strain gage would provide if mounted anywhere on the housing - the read out of the device - everything is completely indistinguishable between considering it to be experiencing balanced forces with gravity applying a "force" to the housing and the mass, and considering it to be uniformly accelerating in space by the action of the one and only force on the housing. In fact, this is the deviation-from-a-geodesic-path view which Bezaire first mentioned. But then again, he's one of those physicist types so I would resist this version on that account alone, if I were you :) > >It's only this small (but significant) difference that made your > >statement not quite right. If we include the phrase "exclusive of > >gravity", then what you said is correct. > > Exactly. Just that from my point ogf view it was your statement that > was "not quite right".(B-) > > >> , but we get a reading of 1 G (or > >> >as you indicated below, -G if you like). But when in freefall, the > >> >net force on the housing is not zero > >> > >> And just what is this net force? > >> > >> , and yet we get a zero reading. > >> > > >> >I think it works this way -- We can think of it as measuring the > >net > >> >force on the housing, *exclusive of gravity*. I think that will > >work > >> >anywhere under any conditions. That's why these devices are > >usually > >> >considered *not* to measure gravity. > >> > > >> >So, the rule is it measures the net force on the housing (in the > >> >sensing direction, of course) exclusive of gravity. > >> > > >> >Bezaire's statement that it measures the housings deviation from a > >> >geodesic path is correct too. But who knows what in the world > >> >"geodesic path" means? I'll think I'll stick to the KISS > >definition. > >> > > >> >> Yes, F=ma, but the measured 'a' must be vectorally added to the > >> >local > >> >> gravitational 'a' to get the 'a' that affects the velocity and > >> >position > >> >> of the housing. > >> >> > >> >> >Well, that's sort of the puzzle. When placed on a table, with > >its > >> >> >sensing axis oriented vertically, there is no difference in > >> >> >acceleration between the two parts. But the reading will be a > >G. > >> >> > >> >> I think it will actually be -G and because we know it is not > >moving > >> >this > >> >> gives us a measure of the local 'a' as G. > >> >> > >> >> [snip] > > -- > Don Cates "he's a cunning rascal" (PN) >