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Doubt !

  1. Oct 7, 2009 #1
    In the mid term exam there was this question ,
    Draw the position - time graph of a body moving with zero velocity.
    I drew the graph (parallel to x-axis) and got full marks . And one of my friend drew the graph (coinciding x-axis) but , he got only 0.5 marks out of 2 . When he asked the teacher, she said that the position = 0 is not defined .But in many of the graphs we start from position = 0.So, what is the mistake in my friend's answer? Isn't his answer correct?
     
  2. jcsd
  3. Oct 7, 2009 #2

    mgb_phys

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    Science Advisor
    Homework Helper

    In a position-time graph any path parallel to the x axis would be correct. The body could have started at 1m away and stayed there.
    In a displacement-time graph (which is how it's normally stated) then it has to be along the axis because the displacement (ie distance from starting point) can't change from zero
     
  4. Oct 7, 2009 #3
    So , my friend's answer is correct , isn't it ?
     
  5. Oct 7, 2009 #4
    If the problem was as you stated, and did not elaborate, then it seems to me the teacher might be unreasonable. If the body happens to be at x=0 with zero velocity, then its path will be on the x axis. If the problem doesn't also ask you to specify the starting coordinates of the body, then your friend seems to have answered it correctly. It might also depend on whether or not your teacher had told the class that the starting position should be stated along with the position-time graphs, and that the necessity of doing so in exams was implied.
     
  6. Oct 7, 2009 #5

    Doc Al

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    Staff: Mentor

    The problem with your friend's answer is that it presumes a specific position, x = 0, whereas a more general diagram with unspecified x-coordinate does not. To his credit, his drawing does represent something with zero velocity; it's just not general enough. Depending on the exact wording of the question, I would give partial credit (perhaps full credit, if the wording was sloppy). (I, of course, would phrase the question more carefully.)
     
  7. Oct 7, 2009 #6

    Dale

    Staff: Mentor

    Unlabeled graphs are a "pet peeve" of mine. I assume the same is the case with your teacher and that your friend has probably been aware of that fact during the course of the class.
     
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