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Thread: Horrible error in a study guide

  1. #1
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    Horrible error in a study guide

    I'm taking the Praxis II General Science exam later this morning, so I bought a study guide (because of the draconian provisions of the DCMA I'm mentioning neither the title nor the publisher)

    One of the questions was, to paraphrase, "why are astronauts weightless?" The correct answer was, again paraphrasing, "because they're so far from Earth." <rude noise>

    I saw this when I was working through their sample test, jumped back in my chair and burst out "...the ....." which made the other people in the room (faculty lunch room; I was substituting) take notice.

    Needless to say, when I get back from the test, I'll be sending them a less than complimentary note.
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    I am taking a Spanish class and the textbook has some remarkable typos. I say remarkable because it is rather sad to make one mistake in Spanish, then make a completely different mistake in the associated English text.

    "Le le, teh teh" could be laughter but due to the context, I doubt it.

    My teacher is seriously considering authors/publishers because the newest edition has more typos that our current version.
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    Quote Originally Posted by swampyankee View Post
    Needless to say, when I get back from the test, I'll be sending them a less than complimentary note.
    OK I'll admit I donīt know what you are talking about. Are you suggesting there is a grammatical error, or just appalled at the seriously inadequate physics in the answer?

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    Quote Originally Posted by Perikles View Post
    OK I'll admit I donīt know what you are talking about. Are you suggesting there is a grammatical error, or just appalled at the seriously inadequate physics in the answer?
    I wouldn't call it inadequate, I would call it a serious error!

    Astronauts on the ISS are probably less than 5% farther from the center of the earth than we are--and the force of gravity diminished by only twice that.

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    You would be weightless if you were out in deep space and too far from any stars or planets to be affected by their gravity, but that's not why astronauts on the space station are weightless.

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    Quote Originally Posted by grapes View Post
    I wouldn't call it inadequate, I would call it a serious error!

    Astronauts on the ISS are probably less than 5% farther from the center of the earth than we are--and the force of gravity diminished by only twice that.
    Yes, but in the absence of a specific location, you are surely entitled to assume far enough away from Earth for the gravitational field to be negligible. That would be an inadequate answer. If you assume the ISS, then, yes, a serious error.

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    Quote Originally Posted by Perikles View Post
    Yes, but in the absence of a specific location, you are surely entitled to assume far enough away from Earth for the gravitational field to be negligible. That would be an inadequate answer.
    but but you don't have to be weightless if you're far from the earth, you could be in another gravity field (on the moon for example), or experiencing acceleration (because of rockets, or possibly some centripetal force).
    If you assume the ISS, then, yes, a serious error.
    that's been the limit for probably over 99% of the astronauts' weightless experiences though. Regardless, the distance from the earth has almost nothing to do with it, because it doesn't imply weightlessness necessarily.

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    The question is specifically about weightlessness in orbit. Oh, and I think I passed the exam, if my explanation of why calcium chloride and potassium nitrate form conducting solutions in water while sucrose doesn't is vaguely correct.
    Information about American English usage here and here. Floating point issues? Please read this before posting.

  9. #9
    Quote Originally Posted by Perikles View Post
    Yes, but in the absence of a specific location, you are surely entitled to assume far enough away from Earth for the gravitational field to be negligible. That would be an inadequate answer. If you assume the ISS, then, yes, a serious error.
    But actually, you aren't entitled to do that. The incorrect answer they provided is true in some cases but untrue in others. But the other answer ("because they are in free fall") is true in all cases, so I would say it's the correct answer.
    As above, so below

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    Quote Originally Posted by Jens View Post
    But actually, you aren't entitled to do that. The incorrect answer they provided is true in some cases but untrue in others. But the other answer ("because they are in free fall") is true in all cases, so I would say it's the correct answer.
    Of course, that's the answer they didn't include.
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    Question and answer leads to incorrect generalization:

    Q: I am standing on a neutron star, am I weightless?
    A: Yes, "because I am so far from Earth."

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    Quote Originally Posted by Perikles View Post
    OK I'll admit I donīt know what you are talking about. Are you suggesting there is a grammatical error, or just appalled at the seriously inadequate physics in the answer?
    I wouldn't consider either a minor grammatical error or a typographical error (in another part of the guide, they referred to "Nobel," vs "noble" gases) serious enough to bring it up the BAUT Forum. I'm appalled at the physics in the answer the guide claimed was "right."
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    Quote Originally Posted by grapes View Post
    but but you don't have to be weightless if you're far from the earth, you could be in another gravity field (on the moon for example), or experiencing acceleration (because of rockets, or possibly some centripetal force). that's been the limit for probably over 99% of the astronauts' weightless experiences though. Regardless, the distance from the earth has almost nothing to do with it, because it doesn't imply weightlessness necessarily.
    Weight itself requires definition. Several posters suggest that bodies in free fall are weightless, yet I doubt that most native speakers of English would accept that their weight changes when they, for example, jump off a chair. If weight is the force acting on a body due to gravity then objects in free fall are not weightless. If weight it the force something else (such as the ground) exerts on a body to keep it at a constant gravitational potential, then bodies in free fall are weightless.

  14. #14
    Quote Originally Posted by swampyankee View Post
    In another part of the guide, they referred to "Nobel," vs "noble" gases.
    That would not necessarily be an error, if they were talking about the speeches people give at the award ceremony.
    As above, so below

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    Quote Originally Posted by Jens View Post
    But actually, you aren't entitled to do that. The incorrect answer they provided is true in some cases but untrue in others. But the other answer ("because they are in free fall") is true in all cases, so I would say it's the correct answer.
    I don't see why not, to be honest. Without specifying a location in space, I think I am entitled to assume a location out of range of any significant gravitational field. This is the condition of almost all of space. I donīt see why I should be expected to assume the ISS or anywhere else.

    Quote Originally Posted by swampyankee View Post
    I wouldn't consider either a minor grammatical error or a typographical error .. serious enough to bring it up the BAUT Forum.
    I agree, but was totally baffled by the OP with

    Quote Originally Posted by swampyankee View Post
    I ... jumped back in my chair and burst out "...the ....." .
    which made me wonder whether there was some issue with 'Earth' and 'The Earth'. Obviously not. Though there is a BAUT thread on the definite article.

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    Quote Originally Posted by whimsyfree View Post
    Weight itself requires definition. Several posters suggest that bodies in free fall are weightless, yet I doubt that most native speakers of English would accept that their weight changes when they, for example, jump off a chair.
    Nor would they say they "lost weight" by taking a space trip.
    If weight is the force acting on a body due to gravity then objects in free fall are not weightless. If weight it the force something else (such as the ground) exerts on a body to keep it at a constant gravitational potential, then bodies in free fall are weightless.
    The question from the OP used the term "weightless"--there's only been a handful of times in space flight history when their answer would even apply--and the majority of that time, they were on the moon where they weren't weightless (now that I've said this, I'm not sure it's true--some of the on surface time is offset by the astronaut left behind)

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    Quote Originally Posted by Perikles View Post
    I don't see why not, to be honest. Without specifying a location in space, I think I am entitled to assume a location out of range of any significant gravitational field. This is the condition of almost all of space. I donīt see why I should be expected to assume the ISS or anywhere else.
    The question specified astronauts, which specified a sphere less than half a million km from Earth (no one's been farther away then the Moon), even at that distance the gravitational field of the Earth still gives them an acceleration of about which is definitely not weightless.
    The ISS (and LEO in general) is assumed as that's the only place to find astronauts now.
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    But is the "study guide" sourced from the same people who set the exams?

    If so, there are probably students putting correct answers down but are being marked wrong.

    I mean, you probably put down that sodium choride solution conducts electricity because it forms ions in solution, whereas sucrose does not and therefore its solutions do not conduct very well. That would be a correct response at this level.

    But if the exam markers have a "correct answer" in front of them that sodium is a metal and sucrose is not, you are scr***d.

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    Quote Originally Posted by HenrikOlsen View Post
    The ISS (and LEO in general) is assumed as that's the only place to find astronauts now.
    I hear what you say, but the assumption of the ISS makes the answer very wrong. Therefore, it is reasonable to deduce that the assumption made by the person writing the question was that this is a theoretical or future astronaut in deep space. That's how I read it, but I'm always in a minority, usually of one.

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    The whole point is that the answer is wrong. Distance has nothing to do with weightlessness.
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    Wait, there is maybe more to this--was this a multiple choice question? What were the other "wrong" answers?

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    Quote Originally Posted by HenrikOlsen View Post
    The whole point is that the answer is wrong. Distance has nothing to do with weightlessness.
    I don't get this. If the astronaut were are a distance of (say) 2 lightyears from the earth, are you saying he/she would not be weightless?

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    Quote Originally Posted by Perikles View Post
    I don't get this. If the astronaut were are a distance of (say) 2 lightyears from the earth, are you saying he/she would not be weightless?
    It depends. If their spaceship is accelerating at 1g, they are not weightless, it's equivalent to standing on the surface of the earth. If they are free-floating around, they are weightless, which they also are when falling from a high building on earth. As Henrik says distance has nothing to do with it, proper acceleration does (which you can have or not have anywhere on earth or in space).

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    The correct answer is: because they are travelling along a geodesic.

    Incidentally this error seems widespread. Back in high school (or whatever the equivalent of when you're 14) our physics teacher said the exact same thing, that astronauts were weightless because there was no gravity in space. When i then asked "then why doesn't the moon fly away?" i got thrown out of class for being difficult to the teacher, go figure..

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    Quote Originally Posted by caveman1917 View Post
    It depends. If their spaceship is accelerating at 1g, they are not weightless, it's equivalent to standing on the surface of the earth. If they are free-floating around, they are weightless, which they also are when falling from a high building on earth. As Henrik says distance has nothing to do with it, proper acceleration does (which you can have or not have anywhere on earth or in space).
    Yes, of course, that's exactly what I thought. My question assumed, reasonably, that there was no acceleration of the spacecraft. I find it very strange that you and Henrick both say distance has nothing to do with it, because it obviously has, the gravitational effect being proportional to the inverse square of that distance. I must be missing something here, everyone seems to be making different assumption to me.

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    Quote Originally Posted by Perikles View Post
    I find it very strange that you and Henrick both say distance has nothing to do with it, because it obviously has, the gravitational effect being proportional to the inverse square of that distance.
    Nobody is stating distance has nothing to do with it.

    Even 2 light years from the earth you would still be feeling the force of gravity from our solar system and the galaxy.
    The reason an astronaut in the ISS is weightless is because he is in free fall and accelerating at exactly the same rate as the ISS.

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    Quote Originally Posted by Perikles View Post
    I must be missing something here, everyone seems to be making different assumption to me.
    I think it may be different interpretations of the word weightless.
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    Quote Originally Posted by HenrikOlsen View Post
    Distance has nothing to do with weightlessness.
    Quote Originally Posted by caveman1917 View Post
    As Henrik says distance has nothing to do with it,
    Quote Originally Posted by moog View Post
    Nobody is stating distance has nothing to do with it.
    OK. If you say so.

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    Different "it"'s.

    Distance has nothing to with with weightlessness and nobody is saying distance has nothing to do with the size of gravitational attraction.

    Weightlessness is not lack of gravitational attraction.
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  30. #30
    Quote Originally Posted by moog View Post
    Nobody is stating distance has nothing to do with it.
    A couple people have done so...and they're quite right. On a rogue iceball two light years away, you won't be weightless. On an accelerating spacecraft anywhere, you won't be weightless. No astronauts have even been far enough from Earth to experience an apparently weightless environment due to sheer distance, if they managed to hold a stationary position with respect to Earth. The answer is just plain completely incorrect.

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