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Determining instantaneous velocity from a graph

  1. Oct 14, 2008 #1
    1. The problem statement, all variables and given/known data
    This is how my problem has been presented:
    The position of a rabbit along a straight tunnel as a function of time is plotted in Fig. 2-26.

    http://img134.imageshack.us/img134/8263/graphxt5.th.jpg [Broken]http://g.imageshack.us/thpix.php [Broken]

    Determine its instantaneous velocity at the following times.

    (b) t = 32.5 s

    2. The attempt at a solution

    Attempts are:
    0.49231 m/s
    0.50769 m/s
    0.523077 m/s
    0.53846 m/s
    0.553846 m/s
    0.56923 m/s
    0.57085 m/s
    0.5846 m/s

    The numbers are obtained, though rounded, for x/32.5 s, where x is, and in order:
    18.552625 (obtained by count pixels and some math to get a number, though it failed)

    The problem is probably just because the graph is terrible for this kind of work, due to the fact that the graph came off some different problem that was intended for students to describe the graph, so the numbers aren't very well set.

    These are the numbers I obtained off the few weeks of physics I had but any other input is fine.
    Its also fine to give a guess off the top of your head, so i can try it, but please base it off something.
    Last edited by a moderator: May 3, 2017
  2. jcsd
  3. Oct 14, 2008 #2
    To find instentanious velocity just draw a tangent to the graph at that specific time and calculate the tangents slope
  4. Oct 14, 2008 #3
    So just to be sure, it would be a line from a point (I used 0,0) through tangent to the point at 32.5s (which would be the middle of the line, since it is about 5 pixels wide).

    Also that it would look like this picture, more or less.

    http://img515.imageshack.us/img515/2900/graph2mg6.th.jpg [Broken]http://g.imageshack.us/thpix.php [Broken]

    If so, then I do get a new number, 0.57143 m/s at 18.57 m.

    Thanks for your input.
    Last edited by a moderator: May 3, 2017
  5. Oct 14, 2008 #4
    Yes that would roughly be it. However I think the tangent line should be a bit higher.
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