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Velocity When a Runner Covers Half a Lap

  1. Jan 25, 2009 #1
    1. The problem statement, all variables and given/known data

    A runner covers one lap of a circular track 40.0 in diameter in 63.2 .

    If she covered the first half-lap in 28.7 , what were her average velocity for that half-lap?


    2. Relevant equations

    We use perimeter of a cirle = 2*3.14*20m to get 125.6

    3. The attempt at a solution

    I know that if the runner runs around a full lap, he or she will be at 0 velocity. But at half a lap I tried 62.8/28.7 to get 2.188. Im sure that is the speed and not the answer. I know I am making a small mistake.
     
  2. jcsd
  3. Jan 25, 2009 #2

    PhanthomJay

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    Hint: Average velocity is the change in displacement (a vector) divided by the change in time. So what's the change of the runners dsplacement, both in magnitude and direction?
     
  4. Jan 25, 2009 #3
    No ideas? 1?
     
  5. Jan 25, 2009 #4

    PhanthomJay

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    I gave you the hint. Take it from there.
     
  6. Jan 25, 2009 #5
    the runner ends in the same place he/she starts.
     
  7. Jan 25, 2009 #6

    PhanthomJay

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    After one lap, she does, but not after half a lap.
     
  8. Jan 26, 2009 #7
    haha.
    well apparently i cant read.
     
  9. Jan 26, 2009 #8
    Got it, 1.4 m/s because displacement is the shortest distance. This would be 40 m / 28.7 s.
     
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