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Physics speed homework

  1. Jun 2, 2009 #1
    1. The problem statement, all variables and given/known data
    A person takes a trip, driving with a constant speed of 91.0 km/h except for a 22.0 min rest stop. If the person's average speed is 62.4 km/h, how much time is spent on the trip and how far does the person travel?



    2. Relevant equations



    3. The attempt at a solution
    not sure how to begin
     
  2. jcsd
  3. Jun 2, 2009 #2

    cepheid

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    Re: stumped

    You haven't put anything under the relevant equations. The problem asks you to deduce distance travelled and time spent based on an average speed. What is the definition of average speed?
     
  4. Jun 2, 2009 #3

    dx

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    Re: stumped

    Let T be the total time that he drives (not including the rest stop), and D the total distance that he travels. You know that D/T = 91 km/h. That's one equation.

    What is the average speed in terms of D and T? This will be your second equation.

    You can solve these two equations for D.
     
  5. Jun 2, 2009 #4
    Re: stumped

    average speed= total distance/total time
     
  6. Jun 2, 2009 #5

    cepheid

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    Re: stumped

    Right, and how does the total time relate to the time that he drives (which is T using dx's notation)?
     
  7. Jun 2, 2009 #6
    Re: stumped

    T-22 minutes?
     
  8. Jun 2, 2009 #7

    cepheid

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    Re: stumped

    Why subtraction? Are you saying that the total time for the trip is LESS than the time spent driving?

    If I spend T minutes driving *and* 22 minutes sitting at a rest stop, what is the total time I spend on the trip?
     
  9. Jun 2, 2009 #8
    Re: stumped

    ok so it would be the total time(T)+22. sorry i got that confused. But i am still unsure were to go from there?
     
  10. Jun 2, 2009 #9

    cepheid

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    Re: stumped

    Remember that you have two equations and two unknowns. The unknowns are:

    distance travelled (D)

    time spent driving (T)

    The two equations are:

    driving speed = 91 km/h = distance driven/time spent driving = D/T

    average speed (by *definition*) = 64 km/h = distance travelled / TOTAL time = D/(T+22)

    Two equations, two unknowns. This means that you can get a unique solution for both D and T.
     
  11. Jun 2, 2009 #10

    cepheid

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    Re: stumped

    Here are the equations more cleanly:

    91 = D/T

    64 = D/(T+22)
     
  12. Jun 2, 2009 #11

    dx

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    Re: stumped

    It's better to write 22 minutes as 22/60 hours to keep the units consistent. Then you will get the answers for D and T in kilometers and hours respectively.
     
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