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Finding time from average speed

  1. Dec 10, 2015 #1
    1. The problem statement, all variables and given/known data

    A golfer rides in a golf cart at an average speed of 3.1 m/s for 28s. She then gets out of the cart and starts walking at an average speed of 1.3 m/s. For how long must she walk if her average speed for the entire trip, riding and walking, is 1.8 m/s ?

    2. Relevant equations

    Vave = distance/time


    3. The attempt at a solution

    This question seems simple considering there's only formula at play. I tried splitting up into 2 parts. The first part, the person walks at an average speed of 3.1 m/s for 28s. Distance can be calculated as 86.8 m

    After this I got lost. I know there's a second part where she walks at an average speed of 1.3 m/s. However nothing else is known. And we know her entire average speed is 1.8 m/s

    Any ideas?
     
  2. jcsd
  3. Dec 10, 2015 #2

    Samy_A

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    Let ##t## be the time she walks.
    You can compute the total time of the trip and the total distance of the trip in function of ##t##.
    As you also know the average speed for the entire trip, this will give you an equation in ##t##.
     
  4. Dec 10, 2015 #3
    I didn't quite understand your second line.
    Right now I have 1.8=86.8 + d / 28 + t where 1.8 is the average speed of the whole trip, d is the unknown distance from the second interval, and t is the unknown time from the second interval.

    Im not sure if im on the right track nor where to proceed from here

    EDIT
    where does the 1.3 m/s speed in the second interval come into play?
     
  5. Dec 10, 2015 #4

    Samy_A

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    I assume you meant ##1.8=(86.8 + d) /( 28 + t)##.

    You know that she walked at 1.3 m/s, so that should allow you to express ##d## in terms of ##t##.
     
  6. Dec 10, 2015 #5

    jbriggs444

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    Minor correction: she walks at 1.3 m/s
     
  7. Dec 10, 2015 #6

    Samy_A

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    Thanks, I corrected the typo.
     
  8. Dec 10, 2015 #7

    Samy_A

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    That's the correct question. You haven't used the 1.3 m/s speed so far.
    What relation is there between 1.3 m/s, d and t?
     
  9. Dec 10, 2015 #8
    Ah I think i got it now.

    Right now we have the following: 1.8=(86.8+d)/(28+t).

    If we find that d then we can have an equation solvable for t.

    In interval two we have 1.3 = distance / t
    and this equals to: distance = 1.3t

    Then we can plug in that value as the d and solve for t :D
     
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