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Homework Help: Average speed problem.

  1. Oct 28, 2007 #1
    1. The problem statement, all variables and given/known data

    theres a motorist that travels 30 miles at 60 mph, and another 60 miles at 30 mph.

    2. Relevant equations

    3. The attempt at a solution

    The average speed of the motorist is 36 mph
    how was this computed?

    i know avg speed is distance / chng of time

    however 30+ 60 divided by 60 + 30 is not 36.
    need help.
  2. jcsd
  3. Oct 28, 2007 #2


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    Homework Helper
    Gold Member

    Please write the full question.
  4. Oct 28, 2007 #3
    ....nevermind i think it got it,

    since hes travelling only half the distance of what speed hes going, then he probably only went about 30 minutes, and then the oppositive of that with the other, which is 2 hrs, then distance of 90 / 2.5 hrs is the avg speed of 36 m/s

    thanks anyway.
  5. Oct 28, 2007 #4
    just to, may be, boost up your knowledge:
    can you tell, why vaverage = (v1 + v[2)/2 is not applicable here? {Note, it would give 45mph}
    Also, note that constant speed is provided in either part of the motion.
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