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Homework Help: Err, real easy problem but what'd he(or I) do wrong

  1. Sep 21, 2005 #1
    the problem was that a guy wants to average a speed of 180 km/h while driving 4 laps around a race track.
    after 2 laps, he's only averaged 150, so its asking what the average would have to be so that he does attain his goal.

    So everybody would read the question, put 210, then check the answer in the back of the book and they would assume they are right.

    But from the start i knew it wasnt what it seemed. Since If 1 lap=1 km, then going 180 km/h, it would take you 80 seconds to finish all 4 laps.

    But, going 150, it would take you 48 seconds to finish 2 laps, and then going 210, it would take you roughly 35 seconds. which comes out to ~83 seconds, and not 80.

    So The point is to find how fast you have to go to do a lap in 32, seconds, which comes out to 225 km/h.

    When i first started explain it, no one would believe me, i felt like galileo or something

    I got 210 myself, and upon close scrutinizing I can't find an error in either of our calculations, and all things considered, the inverse ability function probably made me wrong(what's the saying, you know you're a physics major when you can do vector calculus but don't remember long division?)
  2. jcsd
  3. Sep 21, 2005 #2


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    Homework Helper

    You're correct. Can you read my working ?

    Attached Files:

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