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Thread: Physics Question

  1. #1 Physics Question 
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    Well, our tests came and passed and today we received our papers back to confirm that the answers marked were correct.

    This question arose in one of the multiple choice questions:


    The majority of the class is positive that the answer is (D), however, the teacher is adamant that it is (C) as it was the answer given for a past School Certificate examination question and this question was ripped from the 1998 Section 1 Paper (Question 11).

    Now, personally, I also think that the answer is indeed (D) because surface area remains constant between both diagrams and does not change.

    So, could you please provide the answer that you come up with and how you reached that conclusion, thanks.


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  3. #2  
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    Arrgh, multiple choice. Not very educational. Only good for the lazy teacher. But anyhow:

    I think that nearly all answers can be misunderstood. Do B) and C) refer to the difference between vacuum and air? As long as you regard only one medium in which these experiments are carried out, both answers are wrong. But if you compare vacuum with air, I would assume that all four answers are equivalent.

    Answer A) mentions heavy objects (= objects that have a mass?). From this follows that these objects also have a volume that interacts with the medium. Naturally, air produces friction and breaking that in comparison with a vacuum reduces the acceleration. Is this statement the same like they "fall faster"? Well, they accelerate faster. Coming to B) and C), the shape of an object also defines its surface area. But which one? Its full surface area or the one that points to the direction of the fall? If the latter is the case, then both answers are equivalent. Answer D) is just the more detailed explanation why different shapes and surfaces react differently to friction with a medium. Is says "increased air resistance". Increased relative to what? The vacuum? Vacuum has no air and hence cannot produce any air resistance.

    If I was a pupil and had to pick only one of these four answers, I would heavily complain and choose D).


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  4. #3 Re: Physics Question 
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    Quote Originally Posted by Daniel_C
    Well, our tests came and passed and today we received our papers back to confirm that the answers marked were correct.

    This question arose in one of the multiple choice questions:


    The majority of the class is positive that the answer is (D), however, the teacher is adamant that it is (C) as it was the answer given for a past School Certificate examination question and this question was ripped from the 1998 Section 1 Paper (Question 11).

    Now, personally, I also think that the answer is indeed (D) because surface area remains constant between both diagrams and does not change.

    So, could you please provide the answer that you come up with and how you reached that conclusion, thanks.
    What the questions and answers illustrate is a lack of understanding of the underlying physics and science in general on the part of whoever wrote the question.

    ALL of the answers are true.

    Heavy objects do fall faster in a vacuum than in air. (This is true of all objects, so it is certainly true of heavy ones even though "heavy" is not clearly defined).

    The shape of an object does affect how quickly it falls since air resistance at a given speed and air density is determined by the shape of the object and the direction of movement.

    Greater surface area does generally increase air resistance, either from greater frontal area, though the effect is a bit more subtle that just surface area as orientation and shape are also important. But greater frontal surface area does increase air resistance.

    Increased air resistance, or any "resistance" rather by definition slows the fall of an object.

    So this comes down to selecting the "best" answer, which is sometimes what is requested on multiple choice tests. However there is no such thing as a "best" answer, except in the eye of the beholder. This is just a plain bad question. The fact that your teacher insists on anything is a sign of lack of understanding. There is no rational basis for any "insisting" here. While some people say that there is no such thing as a dumb question, there are certainly incompetently formulated test questions (which is as good an example of an dumb question as anything that I can think of) and this is one.
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  5. #4  
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    I would like to take this opportunity to complain about the trend toward using hyeroglyphics where plain English could be used. Why would we want to use a system of writing employed by cave men and ancient egyptians, when we have a modern alphabet? I had a check engine warning light come on in my car, but it didn't say "check engine" it was a crude outline of an engine, which I thought was a transmission.

    Anyway, the multiple choice problem seems to be more about translating the hyeroglyphics than trying to understand the physics. The experimental results could have been presented much more clearly in a few English sentences or a numerical table.

    That said, D is a better answer than C, for the reasons given and because the flat leaf has the same surface area as the on-end leaf but falls slower in air.
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  6. #5  
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    Quote Originally Posted by Harold14370
    I would like to take this opportunity to complain about the trend toward using hyeroglyphics where plain English could be used. Why would we want to use a system of writing employed by cave men and ancient egyptians, when we have a modern alphabet? I had a check engine warning light come on in my car, but it didn't say "check engine" it was a crude outline of an engine, which I thought was a transmission.

    Anyway, the multiple choice problem seems to be more about translating the hyeroglyphics than trying to understand the physics. The experimental results could have been presented much more clearly in a few English sentences or a numerical table.

    That said, D is a better answer than C, for the reasons given and because the flat leaf has the same surface area as the on-end leaf but falls slower in air.
    I think it may be simply due to the fact that cave men apparently understood physics better than some of the people who formulate questions for students.

    I agree wholeheartedly with your abhorrence of hieroglyphics with regard to automobile instrumentation. They are not at all informative. I suggest that "check engine" signals also include the subtitle "you need a new one" since they give little information to the operator who has no idea what immediate action to take if the signal is given some distance from a mechanic with the computer that allows one to decode that inscrutable message.

    I was with a friend who had just such a message appear a day or so ago-- many miles from pavement let alone a computer -- but we managed to conclude that it was only an oxygen sensor and so relatively harmless at least in the short term.
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    Well no, you have to pick the *most* correct answer. Sorry for not being as specific.
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  8. #7  
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    Quote Originally Posted by Daniel_C
    Well no, you have to pick the *most* correct answer. Sorry for not being as specific.
    There is no such thing and the requirement to do so simply proves that whoever formulated the question is incompetent. See earlier post.
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  9. #8  
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    The answer A contains a true statement, but it is not the (rather, a) correct answer to what the drawings illustrate. The drawings do not contain a comparison between the speed or acceleration of the same (or identical) object in air and in a vacuum.

    The idea of choosing "the most correct" among several correct answers, in a scientific context, is strange to say the least.

    Strange - why do I instinctly feel that the English noun "air" needs no article, whereas "vacuum" calls for an "a"? If anything, there can be various kinds of air, while vacuum is one - just empty space.

    Harold, as for your fury at "hieroglyphs" in the car, you seem to be lucky in two ways:

    1) Your eyesight is still juvenile enough to clearly see the road and the dashboard with the naked eyes, or with one and the same pair of eyeglasses;

    2) You speak English, and live in a country where English is universally known.

    Other drivers can be in a different position, and may find iconograms easier to see and interpret.
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  10. #9  
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    Quote Originally Posted by Leszek Luchowski
    Harold, as for your fury at "hieroglyphs" in the car, you seem to be lucky in two ways:

    1) Your eyesight is still juvenile enough to clearly see the road and the dashboard with the naked eyes, or with one and the same pair of eyeglasses;
    The same pair of bifocals, anyway.
    2) You speak English, and live in a country where English is universally known.

    Other drivers can be in a different position, and may find iconograms easier to see and interpret.
    The speedometer is in miles per hour for the American market. Would it be so much harder to have the rest of the dashboard in English?
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  11. #10  
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    Sorry you have to deal with this sh*t Daniel_C. There is a reason why I do not take physics classes anymore even though I am specializing in it But unless you want to bet everything like I am, it would be prudent to just take the pain and deal with these ignoramuses as best as you can.
    Of all the wonders in the universe, none is likely more fascinating and complicated than human nature.

    "Two things are infinite: the universe and human stupidity; and I'm not sure about the universe."

    "Great spirits have always found violent opposition from mediocrities. The latter cannot understand it when a man does not thoughtlessly submit to hereditary prejudices but honestly and courageously uses his intelligence"

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  12. #11  
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    Quote Originally Posted by Cold Fusion
    There is a reason why I do not take physics classes anymore even though I am specializing in it
    This is one of the most astonishing statements that I have ever seen.

    You are kidding, right ?
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  13. #12  
    Geo
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    The correct answer is C because the arrows representing air resistance remain the same. The horizontal leaf has a slower rate of fall. You can't tell from the diagram whether air resistance has changed, D mentions increased air resistance.

    It's not A because the diagram shows the cube falling at the same rate in a vacuum and in air.

    It's not B because the diagram shows the vertical leaf and the cube falling at the same rate, both in a vacuum and in air.

    Read the question carefully!
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    Quote Originally Posted by Geo
    The correct answer is C because the arrows representing air resistance remain the same. The horizontal leaf has a slower rate of fall. You can't tell from the diagram whether air resistance has changed, D mentions increased air resistance.
    On the contrary! The number of arrows indicating the air resistance is different for the three bodies. So, my interpretation is, the air resistance changes. On the other hand, C is either wrong or unclear, because how do you measure the surface area in this diagram? Furthermore, the surface area of the objects does not change, when you compare both situations. What is the definition of the surface area here? Is it the surface area of the entire body, e.g. the surface of all the 6 sides of the cube? Then C is clearly wrong, because the orientation of the leave does not change its surface area. Or means "surface area" the interaction cross section? This is something else.
    Quote Originally Posted by Geo
    It's not A because the diagram shows the cube falling at the same rate in a vacuum and in air.
    Wrong. If you look closely, you see that the cube falling in vacuum (left panel) is deeper in the bottom row than in the right panel. Also, both panels are lacking a scale. Therefore, it is not clear that the distances in both cases are directly comparable. Furthermore, what does the arrow "motion" indicate? Is it the velocity vector? Or is it the direction of the fall?
    Quote Originally Posted by Geo
    It's not B because the diagram shows the vertical leaf and the cube falling at the same rate, both in a vacuum and in air.
    As I said, it is unclear how this option should be interpreted. If you compare air and vacuum, you will notice that the shape does matter. One could even debate for the pressurised experiment, that a horizontal leaf is a different shape than a vertical one.
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  15. #14  
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    Quote Originally Posted by Dishmaster
    The number of arrows indicating the air resistance is different for the three bodies. So, my interpretation is, the air resistance changes.
    The vertical leaf has two arrows, the cube four. They both fall at the same rate. So, that's D out.

    Quote Originally Posted by Dishmaster
    Furthermore, the surface area of the objects does not change, when you compare both situations. What is the definition of the surface area here? Is it the surface area of the entire body, e.g. the surface of all the 6 sides of the cube? Then C is clearly wrong, because the orientation of the leave does not change its surface area. Or means "surface area" the interaction cross section? This is something else.
    Your overcomplicating it. The question and answer is based on the diagram, the experiment.
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  16. #15  
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    Quote Originally Posted by Harold14370
    The speedometer is in miles per hour for the American market.
    If I drove in the US, I'd want my speedometer in miles per hour because that's the unit the speed limits are expressed in. Even though I think in metric.

    Quote Originally Posted by Harold14370
    Would it be so much harder to have the rest of the dashboard in English?
    Do you mean - hard to make or hard to use?

    To make - of course making a dashboard in English is as easy as making it with iconograms. Making it in the 20+ languages of Europe, let alone those of the world, would be harder.

    To use - you'd be surprised how many people cannot understand simplest English words such as "up" or "left" on various devices. So far, household appliances have succeeded in teaching everybody (or almost) the words "on" and "off".
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  17. #16  
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    Quote Originally Posted by Leszek Luchowski
    Quote Originally Posted by Harold14370
    The speedometer is in miles per hour for the American market.
    If I drove in the US, I'd want my speedometer in miles per hour because that's the unit the speed limits are expressed in. Even though I think in metric.
    If you drive in the U.S. you find that your speedometer has two scales, -- both mph and kph so you get two fo the price of one. The mph scale is more prominent.

    Both displays are presented rationally -- without hieroglyphics -- so that any semi-intelligent driver can understand them.
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  18. #17  
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    First note that the actual truth content of the possible answers is completely irrelevant as the question simply asks which conclusions are valid.

    "Heavy" is meaningless. All we can possibly conjecture from the relative sizes is that the cube of lead has greater mass than a leaf, is "heavier" thus. The only object that qualifies for the nonsensical notion "heavy" is therefore the cube. The drawing shows that the cube does fall faster in the vacuum than in air, hence we could arguably conclude A, in this silly context.

    We have two different shapes: leaf and cube. The shape of an object clearly does not determine how quickly it falls since we observe different rates for both shapes in air and the vacuum. The shape might still affect the rate of fall, even to the same extent for two different shapes. Yet to isolate its effect we would have to observe two objects with different shapes falling at different rates with all other possible factors (mass, surface area, air resistance, etc) equal. We do not observe this, hence we cannot conclude B.

    We cannot conclude C as there is no quantitative information on surface area whatsoever. All we know is that both leaves have the same surface area as they are identical, yet we never observe how two different surface areas compare to each other. That two identical leaves fall at different rates in air does therefore not bear on C at all. We could again arguably conjecture from the relative sizes that the cube has a greater surface area than a leaf, and then also find that we cannot conclude C because surface area is clearly irrelevant in the vacuum.

    There is, however, quantitative information on air resistance: absence of arrows means absence of air resistance and more arrows clearly indicate increased air resistance. If you draw a horizontal line through the bottommost point of the rightmost leaf, you find that the upright leaf (unlike the cube and clearly unlike the horizontal leaf) fell at the same rate in air and the vacuum. Hence D does not follow either.

    We can therefore conclude that the question is complete bullshit.
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