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Average Speed question

  1. Aug 31, 2008 #1
    1. The problem statement, all variables and given/known data
    A car travels up a hill at 31 km/h and then down a hill at 59 km/h.
    Find the avg speed.


    2. Relevant equations



    3. The attempt at a solution
    I tried adding the speeds and then dividing by 2 but I guess that's definitely not how to approach this problem.

    How do I go about starting this problem?
     
  2. jcsd
  3. Aug 31, 2008 #2

    Defennder

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    ?? That's all you're given? I thought average speed was total dist/total time.
     
  4. Aug 31, 2008 #3
    Yeah all I'm given is that those speeds are constant and they want the avg speed for the round trip.

    Maybe "round trip" gives something away.
     
  5. Aug 31, 2008 #4
    Anybody?
     
  6. Aug 31, 2008 #5

    Ygggdrasil

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    Assume the car travels x miles uphill and x miles downhill. Find the total time and divide 2x by the total time.
     
  7. Aug 31, 2008 #6

    Doc Al

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    I presume that the distance up the hill equals the distance down the hill. (Without some such assumption, there would be insufficient information.)

    What's the most basic definition of average speed? Use it.
     
  8. Aug 31, 2008 #7
    Ok I get a really small number. Is that legit?

    I assumed the car traveled 10 km up and down and i divided by 20 (2x).
    I got .024603.....
    Is that correct? Or am I doing something wrong?
     
  9. Aug 31, 2008 #8
    The basic definition is total distance/total time. I have no instance of time and no distance either.
     
  10. Aug 31, 2008 #9

    Doc Al

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    Good.
    Use Ygggdrasil's hint: Call the distance x (or D, if you like). In terms of the distance, find the time for going up and the time for going down.
     
  11. Aug 31, 2008 #10

    Ygggdrasil

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    Check the units on your calculation. You'll see where your calculation went wrong.
     
  12. Aug 31, 2008 #11
    I see where i went wrong. I divided the other way around.
    Does 40.666666666....... sound right now?
     
  13. Aug 31, 2008 #12

    Ygggdrasil

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    Yup. Of course, this assumes that the distance up the hill is equal to the distance down the hill, which may or may not be a correct assumption.
     
  14. Aug 31, 2008 #13
    It is correct. Thanks for the help!
     
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