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Kinematics Question -- 2 Runners Racing Each Other

  1. Apr 7, 2014 #1
    1. The problem statement, all variables and given/known data

    Dave and Andrew are racing. Dave running at a constant speed of 20m/s is passed by Andrew. 2.0s after Andrew passes, Dave accelerates at 1.0m/s^2 until he catches up with Andrew 25s later. What is Andrew's velocity (he does not accelerate)?

    2. Relevant equations

    Kinematics Formulas.

    3. The attempt at a solution
    d=vit+1/2at^2
    =(20)(25)+1/2(1)(25)^2
    =812.5

    Dave travels 812.5m.
    v=d/t
    =(812.5)(25+2)
    =30.7m/s

    I'm not sure if the way I did it was correct. Can someone please tell me how to do it. If you think I'm right though can you please tell me. Thanks!!
     
  2. jcsd
  3. Apr 7, 2014 #2
    You seem to have forgotten that 2 seconds went by before Dave accelerated. during these two seconds both runners moved.
     
  4. Apr 7, 2014 #3
    Usain Bolt runs at 12.4m/s but can only sustain that speed for 20 or 30m. So there is something unrealistic about the numbers you are given.

    With that said, looks like you calculated "d" right but missing the distance that Andrew is ahead after the first 2 seconds.
     
    Last edited: Apr 7, 2014
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