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Homework Help: Average speed

  1. Sep 18, 2005 #1
    If the average speed going to town is 40km/hr, and return at an average speed of 60 km/hr. what is the average speed? why is it not 50 km/hr

    Is the answer 30? because he goes 40 km/hr for 1 1/2 hours. etc.
     
  2. jcsd
  3. Sep 18, 2005 #2

    Doc Al

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    Staff: Mentor

    The average speed is found by dividing the total distance by the total time of travel. (Hint: Find expressions for the time for each leg of the trip in terms of its distance and speed.)
     
  4. Sep 18, 2005 #3
    You can pretend the distance going to town is 200km. Now find the total distance of there and back, divided by the total time. So you must calculate the total time.

    An easy way to understand this is pretend you are planning on going from point A to point B(which is 100km from point A). If you go there at 1000km/hr, it will only take you 6 minutes, and if you go back at .00001km/hr, it will take you a LONG time, so doesn't it make sense that your average speed would be quite low?
     
  5. Sep 18, 2005 #4

    HallsofIvy

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    Science Advisor

    You are correct that the "average speed" is not the arithetic average ([itex]\frac{v_1+v_2}{2}[/itex]) but any "average" of two numbers has to be between them!
     
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