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How much time does it take the walker to make the round trip?

  1. Sep 11, 2004 #1
    Two piers, [tex]A[/tex] and [tex]B[/tex] , are located on a river: [tex]B[/tex] is 1500 m downstream from [tex]A[/tex] . Two friends must make round trips from pier [tex]A[/tex] to pier [tex]B[/tex] and return. One rows a boat at a constant speed of 4.00 km/h relative to the water; the other walks on the shore at a constant speed of 4.00 km/h. The velocity of the river is 2.80 km/h in the direction from [tex]A[/tex] to [tex]B[/tex] .

    what does "relative to the water" mean?

    1.) How much time does it take the walker to make the round trip?
    answer must be in mins

    2.) How much time does it take the rower to make the round trip? answer must be in mins

    my work:

    Given:
    x=1500m
    v(b) = 4.00 km/h
    v(w) = 4.00 km/h
    v(r) = 2.80

    t1 = (1500)/(4+2.80) = 220.588secs <--- downstream time
    t2 = (1500)/(4-2.80) = 1250 secs <--- upstream time

    t(t) = 1470.588s <--- total time

    how would i solve #1 ?

    im thinking that i would need the distance formula

    x(t) = x(0) + v(0)t + 1/2at^2
    2(1500) = 0 + 2.80t + 0 (x(0) = 0, because at t=0, x=0. and a=0 because of constant speed)

    3600= 2.80t..... nevermind, it doesnt look right, can someone help?
     
  2. jcsd
  3. Sep 11, 2004 #2

    Doc Al

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    Staff: Mentor

    Basic idea is OK, but you are mixing up units. It's easier than you think. I'll do the first part, the time from A to B:
    t1 = D/V = (1.5 Km)/(6.28 Km/hour) = 0.24 hours
    How many minutes is that? You take it from here.

    Exactly the same way, only now the speed is just 4.0 Km/hour both ways. So, t = D/V ...
     
  4. Sep 11, 2004 #3

    HallsofIvy

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    Staff Emeritus
    Science Advisor

    "relative to the water" means that that is his speed treating the water as it were not flowing. The actual or "true" speed is the water's speed added to his boat's speed when he is going down stream with the current, and subtracted from his boat's speed when he is going upstream against the current.
     
  5. Sep 11, 2004 #4
    thank you, works perfectly
     
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