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Average velocity

  1. Aug 10, 2011 #1
    1. The problem statement, all variables and given/known data

    A family travels from hither to yon one afternoon. On this day, the driver spends half the distance traveling at 77 km/hr and the remaining half at 50 km/hr. What is the average speed during the trip?

    2. Relevant equations



    3. The attempt at a solution

    I assumed that the trip was 2 hours.

    If each speed is half the distance of the total trip, then 77 km/hr lasted for .82467 hours, and 50 km/hr lasted 1.27 hours.

    I then found a weighted average, [77(.82467) + 50(1.27)] / 2

    For 63.5 km/hr, which is incorrect.

    What part of my process is wrong?
     
  2. jcsd
  3. Aug 10, 2011 #2

    SteamKing

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    Assuming a certain amount of time (2 hours) is your problem. You know that half the distance is covered at one rate, and the other half at a different rate.

    (Hint: Use the formula distance = rate * time)
     
  4. Aug 10, 2011 #3
    I don't understand how to apply D = rt, I only know rate.

    I really have no idea what to do unless I assume some total time, and I didn't think it would matter..
     
    Last edited: Aug 10, 2011
  5. Aug 10, 2011 #4

    PeterO

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    Obviously you can't assume a total time - it gives you the wrong answer.

    Try assuming a total something else.

    EDIT: If the family trip was to visit someone, and come home again what would that tell you?
     
  6. Aug 10, 2011 #5
    Thanks guys, I think I got it.
     
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