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Average Speed Problem

  1. Aug 31, 2009 #1
    1. The problem statement, all variables and given/known data

    You plan a trip on which you want to average 97.2 kilometers per hour. You cover the first half of the distance at an average speed of only 72.5 kilometers per hour. What must your average speed be in the second half of the trip to meet your goal? Hint: Pick a distance.

    I have tried this twice on webassign and gotten the wrong answer (I only have 3 submissions so i must make sure this is 100% right.

    All answers need to be rounded to 3 sf's (only at end)


    2. Relevant equations

    V=d/t

    3. The attempt at a solution

    So i attempted to say your total distance traveled was 145km...

    I then said that you traveled 72.5 km in one hour, and you next want to cover the last 72.5 km in x km/h in order to arrive at 92.7 km/h...

    I next found the difference between the speed going half way and the wanted average speed (92.7) km/h..... I then found the difference to be 20.2 which I added onto 92.7 to get 112.9 km/h

    So would 112.9 km/h be my answer?

    (or 113 with 3 sf's)

    This is my first assignment for AP Phys B so bear with me (I am new :))
     
  2. jcsd
  3. Aug 31, 2009 #2

    Redbelly98

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    No, the answer would not be 112.9 or 113 km/h. We must use the definition of average speed,

    avg. speed = distance / time​

    Hint: If 145 km is the total distance travelled, what must the total trip time be in order to average 92.7 km/h?

    From that, get the time of the return trip only and take it from there.
     
  4. Aug 31, 2009 #3
    So...

    (1 hr/ 92.7 km)=(145 km/ x hours)
    x hours = ~1.5641...

    So first 75 km is covered in 1 hr which leaves us with ~.5641 for the second half of the trip...

    so 75km/~.5641 hrs
    ~= 132.935 km/h
    or 133 km/h?
     
  5. Aug 31, 2009 #4

    Redbelly98

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    You have the right idea, but it is 72.5 km and not 75 km right?
     
  6. Aug 31, 2009 #5
    yes so 128.5 .... 129

    correct?
     
  7. Aug 31, 2009 #6

    Redbelly98

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    Yes, looks good :smile:
     
  8. Aug 31, 2009 #7
    alright im submitting.... :/

    wrong answer... :( :(

    8/10 on assignment
     
  9. Aug 31, 2009 #8

    Redbelly98

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    Sometimes these online physics sites do intermediate rounding of calculations, even though they instruct you to do that at the end. They could very well think 128 km/h is the answer.

    Looks like you have used up your 3 tries though :frown:
     
  10. Aug 31, 2009 #9
    yep this is why i HATE webassign.... :(

    NVM- I redid it and it came out to 147 km/h....

    guess i got my calc's wrong... :( gosh this is annoying...
     
    Last edited: Aug 31, 2009
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