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Displacement and Velocity Problem

  1. Sep 11, 2009 #1
    A car making a 100-km journey travels 40 km/h for the first 50 km. How fast must it go during the second 50 km to average 50 km/h?

    2. Relevant equations

    3. The attempt at a solution
    I calculated the time to travel the first 50km at 40km/h, to take 1.25 hours. So it take 1.25 hours to drive 50km at 40km/h. So I need to find how fast it must go during the second 50km to average 50km/h. I am confused. Is this a two-part problem?
  2. jcsd
  3. Sep 11, 2009 #2


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    Hi Loppyfoot! :smile:

    Well, it's a two-part solution :wink:

    first find what the total time must be to average 50 km/h, and then find how fast the last half has to be.

    What do you get? :smile:
  4. Sep 11, 2009 #3
    Alright, I think I have seemed to have gotten it; with the little catalyst you provided.
    If the first half take 1.25 hours, the last half must take 0.75 hours. I then used d=rt, and plugged in:
    50km = r(0.75hours)
    r = 66.67 km/hour


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