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Ill-posed question.

  1. Jul 4, 2009 #1
    I though you might like a light break to solve a problem probably more interesting than the author intended. This is from Physics, Classical and Modern, by Gettys, Keller, and Skove, 1989 (first?) edition. Question 10-1, page 220.

    Estimate the maximum magnitude of momentum you have ever had. In what reference frame?

    This is a text for a 2 or 3 semester physics survey course, so the typical reader may be unaware of some reference frames available for answering the question.
  2. jcsd
  3. Jul 4, 2009 #2


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    Meh. Unless you want to be unreasonably nitpicky (e.g., a blowhard) it's abundantly clear that the author means the only frame of reference which matters to 99.999% of people, which is relative to the Earth's surface.
  4. Jul 4, 2009 #3


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    If I'm allowed to interpret "maximum" as "supremum", then my answers to the two questions are "[itex]\infty[/itex]" and "none":smile:. These are exact solutions, not estimates.

    If I'm not allowed to do that then there is no exact solution.
  5. Jul 5, 2009 #4
    So, how many points would you take off for those students with more imagination and sense of humor than the book's authors?
  6. Jul 5, 2009 #5


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    Did the words "In what reference frame?" appear in the text book question, or were they added by jimmysnyder? If they appeared in the question, I think that was an intentional clue to get students to think of alternatives. I very much doubt the authors intended my response in post #3, but they may well have been inviting students to consider a centre-of-earth frame or a sun-centred frame (for example).
  7. Jul 5, 2009 #6
    In the book. Here's another, from the first chapter on units of measurement. Question 1-4, page 10.

    If you were told that everything in the universe had expanded to twice its former size while you slept last night, how would you check to see if it was true? What if all clocks suddenly ran at half speed? What if the mass of everything doubled? What if all those things happened simultaneously?

    "Size" is not defined in the book, so it is unclear whether the authors mean length or volume, but volume makes more sense. Also unclear is whether students are allowed to apply what they know about the stability of the atom after having read about m, s, and kg.
    Last edited: Jul 5, 2009
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