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A person travels by car from one city to another with different constant speeds

  1. Oct 15, 2010 #1
    A person travels by car from one city to another with different constant speeds between pairs of cities. She drives for 30.0 min at 80.0 km/h, 12.0 min at 100 km/h, and 45.0 min at 40.0 km/h and spends 15.0 min eating lunch and buying gas. (a) Determine the average speed for the trip. (b) Determine the distance between the initial and final cities along the route.





    I try to solve



    (a) Averge speed = total distance / total time



    = 30 + 12 + 45 + 15 / 80.0 + 100 + 40.0 = 0.1673 km/h



    (b) The distance

    = 80 + 100 + 40 = 220 km




    please I wan t to correct my answer .
     
  2. jcsd
  3. Oct 15, 2010 #2

    Borek

    User Avatar

    Staff: Mentor

    This is wrong. How far do you get driving 30 minutes at 80 km/h?
     
  4. Oct 15, 2010 #3
    which one is wrong and I fo same the

    Averge speed = total distance / total time

    ??
     
  5. Oct 15, 2010 #4
    I try to solve agine

    (a) Averge speed = total distance / total time

    now i try to convert all the time to hour
    by using 1 h = 60
    30 /60 = 0.5 and 12/60 = 0.2 and 45/60 = 0.75 and 15/60 = 0.25

    0.5 + 0.2 + 0.75 + 0.25/ 80.0 + 100 + 40.0 = 1.4511 km/h




    (b) The distance
    = Distance = velocity / time =
    1 ) 40 - 12 / 1.95 = 14.35 m/s
     
  6. Oct 15, 2010 #5
    where are you ?
     
  7. Oct 15, 2010 #6
    you know
    Averge speed = total "distance" / total time

    are you sure that's what you have done in your solution?
     
  8. Oct 15, 2010 #7

    Borek

    User Avatar

    Staff: Mentor

    I drove my wife to the doctor and later we went shopping. You see, I have a life outside PF.

    Summing speeds is not a way of calculating distance.

    The only thing you approached correctly so far was calculation of total time.

    No idea why you try to calculate average speed dividing time by the distance, instead of distance by the time.

    To calculate total distance you have to separately calculate distance covered with each speed, then sum them. For example, if you drive for half an hour at 40 km/h and then for 15 minutes at 100 km/h, you cover 0.5h*40km/h=20km in the first leg and 0.25h*100km/h=25km in the second leg, total of 45 km.
     
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