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Vectors around a circular track

  1. Oct 21, 2004 #1
    A jogger runs around a 2 km circular track as shown below. Using graph paper, draw a displacement - time graph of the continuous North vector which runs from the start axis to the jogger's current position. Identify the shape of your graph. Draw a corresponding distance - time graph for the jogger's, overall distance covered. Label important points between the 2 graphs.

    circular distance time
    (m) (s)
    0 0
    250 60
    500 130
    750 185
    1000 245
    1250 303
    1500 366
    1750 437
    2000 501

    also included is a circle with a perp. line inside of it
    . The point on the right side of the circle is labeled start/finish. Any clue on how to do this?
  2. jcsd
  3. Oct 22, 2004 #2


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    Staff Emeritus
    Science Advisor

    You are told that the track is a "2 km circular track" so I assume that "2 km" is the length. I suspect that this problem is preparation for "harmonic motion" and that you are not expected to do detailed calculations with trig functions.

    Here's what I would do. Since the circle given is not actually 2 km long (!) figure out how long it really is (measure its diameter and multiply by pi) 2km/actual length is the factor you will need. I might actually take a strip of paper as long as the circle you are given and mark off, say, 1/4, 1/2, 3/4, 1, 1 1/4, etc. km on it. (since the track is actually 2 km long, 1/4 km will be 1/8th of your strip of paper, etc.)

    Now use your strip of paper to actually mark the postions given on the circle. 0 m will, of course, be whatever starting point you want (perhaps on the right would be best), 250 m= 1/4 km will be 1/8 of the way around the circle, 500 m= 1/2 km will be 1/4 of the circle, etc.

    NOW measure just the "height" of each of those marks from you starting point (if you made you starting "on the right", that is horizontally with the center of the circle, you will be measuring only the vertical distance. That's "north". Now make a graph using the time values you are given as the x-axis and the corresponding vertical distances you have measured as the y-axis. That will be a very rough graph of course, but you should be able to recognize what type of function it corresponds to (I gave you a hint above).
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