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Seesaw balancing

  1. May 17, 2012 #1
    Hi

    I thought if there is a heavier person and a lighter person the heavier person would sit nearer the fulcrum to make the seesaw balance as weightxlength must equal. However on this question 'which picture shows how the two boys will balance better?' the answer is the one with the bigger person further from the fulcrum and the smaller person nearer to it.

    Why is this?
     
  2. jcsd
  3. May 17, 2012 #2

    sophiecentaur

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    It isn't. The principle of moments rules, as everywhere else.
     
    Last edited: May 17, 2012
  4. May 17, 2012 #3

    sophiecentaur

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    Meaning what? It's a bit too terse for me, I'm afraid.
     
  5. May 22, 2012 #4
    I will write out the question.

    'Which picture shows how the two boys will balance better'? (If equal, mark C)

    Both diagrams show the fulcrum of the seesaw in the same location.

    Diagram A has the slimmer boy on the far left which is the furthest side from the fulcrum. The larger boy is on the far right, the nearest side to the fulcrum.

    On B it is the exact opposite with the larger boy on the far left and furthest from the fulcrum and the slimmer boy on the far right, so nearer to the fulcrum.

    The correct answer is shown as B. So the book is saying that for the two boys to balance better the larger boy has to be further from the fulcrum than the slimmer boy.

    Anyone explain this please?
     
  6. May 22, 2012 #5

    Nugatory

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    If you're quoting the book accurately, the book is wrong.
     
  7. May 22, 2012 #6

    sophiecentaur

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    I don't think there is a single textbook in the world that has no mistakes in it. It's a fact of life.
    Sometimes they have been known to put the wrong answer in the back to sort out the students who just copy the answers out. The teacher can then catch them red handed.
     
  8. May 22, 2012 #7
    There is no clear statement of a problem here. The FIRST thing you should have confidence in is your text book, there are mistakes it is true but they tend to be very rare in text books.
    Text books have to sell, opinions in forums are just that.... opinions.
    Lets see the text book before we doubt it.
    A teachers job is not to catch 'them red handed'
     
  9. May 22, 2012 #8

    K^2

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    No. Errors in books are common. You should have confidence in people who understand the subject. A person can make a mistake, but a person can reflect on that mistake and correct it. A printed mistake stays a mistake until it is caught, sent to publisher, forwarded to editor, and fixed in following editions.

    It would be nice to see a scan/picture of the problem in the book, but if description is correct, book is wrong.
     
  10. May 22, 2012 #9

    sophiecentaur

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    Au contraire. It is one of the functions of a teacher to point out that you get nowhere by cheating in that way. It's a waste of the student's time but so many of them try it on. I'm not saying it's a battle with the aim to make them feel small. It can always be dealt with in a light hearted manner but It shows them that they can't take things for granted and that the point of an assignment is to actually learn something.

    My (broad brush) assessment of text books also applies to GCSE and A Level Exam Papers. Each one, almost without fail, has at least one Naff question in it with no proper answer.
     
  11. May 23, 2012 #10
    Books where 'they have put wrong answers in the back to sort out students who just copy the answers' sounds interesting.
    I have never met this, does anyone have any reference for such a book, I would love to own one.
    Are such books available to the general public and kids who don't have teachers.....how would they know about this practice?
    The best book I use gives answers to alternate questions. Mistaken answers can be put down to misprints and they are not that important. It is errors in physics knowledge in text books that would be worrying.
    The worst I have seen (only 2!!) are the explanation that the unsharp shadow caused by the Sun was due to diffraction and phasor diagrams in AC theory described as rotating clockwise.... not 'wrong' but unconventional and therefore very confusing for students.
     
  12. May 23, 2012 #11
    I have just read the rules of physics forums and it states there that all posts should be traceable to STANDARD TEXT BOOKS.
    The originators of these forums seem to have a healthy respect for physics text books.
    This is a very good principle, after all text books are what is available in every educational establishment. You cannot write them off in an easy way.
    They are written by 'people who understand the subject' (note K^2)
     
  13. May 23, 2012 #12

    sophiecentaur

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    Who's "writing off" textbooks? Where would we be without them?
    The fact is, however, that nearly all text books have errors in them. These errors have been know to propagate from book to book. A level and beyond tend to be a bit better but the elementary books often have sections which are written by relatively uninformed non-specialists. You cannot guarantee that they do "understand the subject" and you often need to take it on trust. The diagrams can be particularly bad because these used to be drawn by non-technical draughtsmen and some really glaring errors can get through the proof reading process.
    Text books should be read with the same caution that should be applied to any other source of information.
     
  14. May 23, 2012 #13
    'Text books should be read with the same caution that should be applied to any other source of information'...... including teachers?????
    Do you not think that it would be a good idea that ideas put forward here were backed up with some reference, Wiki is very popular but it is no more a teaching/learning resource than is a dictionary for the English language.
    There is some very bad physics put forward here, nightly, as explanations that can be easily found in standard text books..... who moderates this release of information?
     
  15. May 23, 2012 #14

    K^2

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    Do you really need us to bring in sources on moments of forces? This is high school level physics. Both me and sophiecentaur would probably be more than qualified to write a high school mechanics text. But I do have Goldstein sitting around here somewhere. I'm sure I can dig up some quotes from it on moments that directly contradict the problem as stated by the OP. It's just not really necessary. That's rather trivial, low level knowledge, and a clear typo in the text the OP is quoting, provided that he's quoting it correctly.
     
  16. May 23, 2012 #15
    Well, let's not criticize the textbook author too much. We haven't seen the problem and most likely the problem is not with the diagram or the author's knowledge of physics, but a simple typo of A vs. B. I know I've made my share of typos.
     
  17. May 23, 2012 #16

    DaveC426913

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    Whether we believe textbooks are above error or not is irrelevant.

    There are exactly two possibilities here:

    The book is exactly as described, and is wrong.
    The OP has incorrectly described either the problem, the diagram or the answer.

    We cannot know without seeing a scan of the book for ourselves. (For all we know, the OP has dysgraphia, or has a laterally-inverted copy of the real diagram.)
     
  18. May 24, 2012 #17

    K^2

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    I don't think anybody tried to. If the OP's description is correct, it's almost certainly a trivial typo.
     
  19. May 24, 2012 #18

    DaveC426913

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    Well, it's not a trivial typo if it resulted in exactly the wrong answer.

    BTW, it's not the author at fault here; it's the editors at the publishing company. You can bet it goes through quite a few hands, being checked for accuracy by knowledge-experts.

    (Ironically, I am right ii the middle of editing a school textbook. Got to check every answer to every math question!)
     
  20. May 24, 2012 #19

    sophiecentaur

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    Let's face it chaps. The book in which this question appeared wasn't Degree Level. Text book writers do it more for love than for money and the answers in the back are the last thing we should be worrying about unduly. It's not as if they'd printed the speed of light as 3X10^9m/s!
    (You have my sympathy, Dave and I hope you spot all the errors before the first printed copy hits your front door mat.)
     
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