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Homework Help: One Dimensional Kinematics

  1. May 12, 2012 #1
    1. The problem statement, all variables and given/known data
    "A race car driver must average 180 km/h for 4 laps to qualify for a race. Because of engine trouble, the car averages only 150 km/h over the first 2 laps. What speed must be maintained for the last 2 laps?"

    2. Relevant equations

    3. The attempt at a solution
    So I do know the answer to be 225km/h but I don't quite understand why. Since half the time trial is completed with the driver being 30km/h below the needed average, wouldn't the 2nd half just be 30km/h above the needed average (210km/h for the last 2 laps)? I'm missing something here.
  2. jcsd
  3. May 12, 2012 #2
    You r averaging the speed for individual laps.. (150 + v)/2 = 180, and thus v=210

    Actually avg. speed = total distance/total time = (150*2d + v*2d)/[2d/150 + 2d/v] = 180

    In short you r assuming that he takes equal time to complete the first 2 laps and the last 2 laps, which is incorrect.
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