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Homework Help: Reading the position - time graph:

  1. Jun 26, 2007 #1
    1. The problem statement, all variables and given/known data
    A physics proffessor leaves her house and walks along the sidewalk toward campus. After 5 minutes, it starts to rain, and she returns home. Her distance from her house as a function of time is shown in the figure below.(ignore the paintbrush just assume that its a curved line)
    At which of the labeled points is her velocity...
    a.) zero?
    b.) constant and positive?
    c.) constant and negative?
    d.) increasing in magnitude?
    e.) decreasing in magnitude?

    2. Relevant equations

    3. The attempt at a solution
    a.) zero?
    none.... i think zero is from the end after IV

    b.) constant and positive?

    c.) constant and negative?
    point V

    d.) increasing in magnitude?
    from points I - II

    e.) decreasing in magnitude?
    from points III - IV
    Last edited: Jun 26, 2007
  2. jcsd
  3. Jun 26, 2007 #2


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    Staff Emeritus
    Science Advisor

    Was that graph given to you in the question, or have you drawn it from a previous part?
  4. Jun 26, 2007 #3
    You should be right (I think)
    0 velocity exists at 0 distance (say displacement/deltaT)...velocity is zero seemingly only at limt = 0 and when her journey ends. 0 velocity also exists when limit of the point is a tangent line parallel to the x-axis...so...
    a)Not an Optional Point
    b)No Point
    c)Point V
    d)Point I and Point II
    e)Point III and Point IV
    -----------------Same Answers as Yours----------------------
    (I'm just an amateaur at the moment, so I do not know if this will be of any help)
  5. Jun 26, 2007 #4
    i have no scanner but i think this is likely similar to the graph in my book
    and the brush ... its represented as curves
    and thanks Gear300 for your opinions
    Last edited: Jun 26, 2007
  6. Jun 26, 2007 #5
    No Problem
  7. Jun 26, 2007 #6


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    Science Advisor

    Unfortunately, I'm not sure Gear300's answers are very helpful- he seems to be thinking of average velocity.

    Yes, you are correct that as long as the graph is going upward, velocity is positive, as long as it is going downward it is negative. At the point where she "turns back", changing from going away from her home (positive velocity) to going toward her home (negative velocity) here velocity is 0. Unfortunately, on your graph, that is not one of the labled points!

    I'm not happy with the wording of B or C: it makes no sense to say that the velocity is "constant" at a single point- "constant" velocity can only apply over a time interval. On a distance-time graph, a velocity will be constant where the graph is a straight line. Yes, the velocity is constant (and negative) on the interval containing V. The velocity is increasing where the graph is "convex up" and decreasing where it is "convex down"- it looks likethose occur at I and III respectively.
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