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Determining Distance

  1. Nov 25, 2012 #1
    Hello everyone,
    I'm taking an online class and my teacher is less then helpful :(
    I'm having trouble with one of the questions and the textbook doesn't have any similar examples. I've solved part A, but I'm unsure how to set up part B. Any assistance would be greatly appeciated!!! Thank you

    So here's the question:
    A person makes a trip by car between 2 cities. A record of the trip follows: 30.0 min at 80km/h, 12 minutes as 100km/h, 45 min at 40km/h and 15 min for lunch break and gas.

    a. determine the average speed for the trip.
    b. determine the distance between the cities


    So for part a, i calculated that the average speed for the trip is 86.9km/h, and i'm happy with that answer.
    but for part b, i'm not sure how to set it up.

    total time for the trip is 102 minutes (or 1.7 hours) but that includes the break.. Is that information about the break even useful or is it in there to mix me up?

    Without the break, the total time of the trip is 87 minutes (or 1.45 hr) but I dont know how to organize it so that I can figure out how far the cities are apart?

    I think I have to include my answer from part A, with the average speed being 86.9km/h.. ANy ideas? Thanks
     
  2. jcsd
  3. Nov 25, 2012 #2

    CWatters

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    Work out the distance travelled during each period (eg 30min at 80km/h = 40km).

    Add them up.
     
  4. Nov 25, 2012 #3
    Intuitiively the average speed that doesn't feel right.

    You say 90 kph average speed --- but you only exceed 90 kph by a small amount, and for only 12 minutes out of over 100 minutes. An average is supposed to be representative of your trip as whole, so does that sound right?

    Making approximations like this is a useful skill to develop in mathematics. Otherise it's very hard to tell if you accidentally press a wrong button on the calculator. I haven't done the calculation but my spidersense is tingling.
     
  5. Nov 25, 2012 #4

    CWatters

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    Good call.

    I would suggest the OP answer part b first, then use the info to answer part a.
     
  6. Nov 25, 2012 #5

    K^2

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    The reason information about the break is included is because it influences average speed.
     
  7. Nov 25, 2012 #6
    Thank you everyone..

    So you think my answer A is wrong? I felt it made sense.. BUT this is my first physics class, and it is self taught....
    This is what i did:
    (80km/h / 30min)+ (100km/h / 12min) + (40km/h / 45 min) then i found a common demoninator and added them up, then reduced, and i got 86.9km/h

    Should I include the break in there?



    So solving part B first, I get a distance of 90km between the two cities
    30 min @ 80km = 40km
    12 min @ 100km = 20km
    45 min @ 40 km = 30km
    Add them up = 90kms distance

    Then going back to part A, is this where I'll include the 15 minute break. If i include the break, then I get an average speed of
     
  8. Nov 25, 2012 #7
    i think i got it.. i get average speed of 62.07. Thanks guys
     
  9. Nov 25, 2012 #8

    K^2

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    That's not how you find averages. It's easier if you answer part B first, then find average velocity from that. Average velocity is distance traveled divided by time it took. You include the break time in the later.

    Better, but you didn't include the break.
     
  10. Nov 26, 2012 #9

    CWatters

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    Yes must include the break. Think of it as a period when they are travelling very slowly!

    So

    total distance = 90km
    total time = 102 mins (1.7 Hours)
     
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