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Homework Help: Ridiculus position-time graph extrapolation

  1. Feb 24, 2009 #1
    I'm working on some physics lab homework and one particular graph is obnoxious. It's hard to tell whether it's intended as a compound curve, or if it's just a piss-poor sketch. I've contacted my professor but he hasn't responded for days, and it's due tomorrow. If I'm not mistaken, the decreasing slope towards the end of the graph would indicate negative acceleration, but the blank AvT graph isn't provisioned for such.

    EDIT - I can't believe I was so stupid as to not mention I'm referring to graph B. Apologies!

    1. The problem statement, all variables and given/known data

    3. The attempt at a solution
    Last edited: Feb 24, 2009
  2. jcsd
  3. Feb 24, 2009 #2


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    Homework Helper

    Welcome to PF.

    It would help if you were more specific about exactly which graph you have a question.
  4. Feb 24, 2009 #3
    Sorry! Would you believe that the whole time I was posting I was telling myself to remember to mention it was graph b? *sigh*, my brain is mush at the moment. Fixed! Thanks for pointing it out.
  5. Feb 24, 2009 #4


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    Homework Helper

    In form, if your question is about the negative acceleration, that is correct. there must be a negative value there.

    But at the start there, it looks like you have an impulse or near impulse in the sudden velocity increase, because of your Δv/Δt.
  6. Feb 24, 2009 #5


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    I would smooth your velocity profile up. To be honest, I'm sure that the graph isn't supposed to look so poorly. To me, it reminds me of an equation that CFD users use quite often:
    [tex] f(x) = 3x^2 - 2x^3[/tex]
    Which smoothly goes from (0,0) to (1,1), while maintaining zero derivative at both points. So, basing my "answer" on that, I would say that the velocity curve should be an "arch" with the acceleration being a negatively linear. However, based on that sketch, there might be some jacked up stuff going on at the beginning.
  7. Feb 24, 2009 #6
    It's frustrating how ambiguous it is. This is a 100-level physics class (introduction to physics), mostly newtonian mechanics using only algebra/trig. As you can see from the other two graphs, this lab is supposed to be pretty simple stuff. Just getting a feel for what acceleration is and how "deceleration" is just acceleration opposite to the velocity. I've returned to school after an 8 year break to get a degree in physics. I took this class to "brush up" before getting into the heavier stuff, but it's causing a lot of frustration with a lot of ambiguous questions. I'm tending to "over-think" a lot of the questions. I may be forced to leave this one blank and hope to catch the professor before class. Thanks for all the help everyone.
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