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Homework Help: Kinematics Problem on river

  1. Jan 22, 2008 #1
    A boat takes 2.6 hours to travel 28.43 km down a river, then 5.5 hours to return. How fast is the river flowing?

    So we want to find the velocity the river has.

    I'm not sure I even started this right.
    First I converted 2.6 hours to seconds = 9360 s
    Then 5.5 hours to seconds = 19800 s
    And 28.43 km to meters = 28430 m

    I then used the linear uniform motion equation (xf = xi + vi*t). Am I right in doing this, there is no gravity, correct?

    28430 = vi*9360s ---> vi = 3.037 m/s
    28430 = vi*19800 ---> vi = 1.436 m/s

    This doesn't seem right at all, I don't think I'm understanding the question completely.
  2. jcsd
  3. Jan 22, 2008 #2
    Is the answer 0.80 m/s @2.88 km/h?
  4. Jan 22, 2008 #3
    Tried it already, incorrect.
  5. Jan 22, 2008 #4
    How about 0.157 m/s?
  6. Jan 23, 2008 #5
    let x be the speed of boat and let y be the speed of river...

    28.43/x+y=2.6 >>x+y=10.93 [downstream]

    28.43/x-y=5.5>>x-y=5.18 [upstream]


    >>x=8.05 km/hr=2.236 m/s

    >>y=2.88 km/hr=.8 m/s

    verify and tell if the answer is correct or not..

    explanation shall be done later....
  7. Jan 23, 2008 #6
    physixguru, I've tried 0.8 m/s as the answer to the velocity of the river and it is incorrect.
  8. Jan 23, 2008 #7
    Hi, im new here but i think ive got the answer to this, this is assuming the boat is travelling at a constant velocity.

    So far the two values youve worked out are the same as mine(im not familiar with the equation (xf = xi + vi*t) i just used v=s/t )

    What weve worked out is the velocity of the boat up and downstream, not the river itself.

    v1 = 3.03 m/s
    v2 = 1.43 m/s

    So would the Resultant velocity of the River not just be this

    vr = v1 - v2

    vr = 3.03 - 1.46

    vr= 1.6 m/s

    Try looking at it like this perhaps

    3.03 m/s

    (Could someone else verify this though? :P)
  9. Jan 24, 2008 #8
    if the answer above is incorrect then...u shall have to apply vector algebra equations to solve the problem...directions... i mean....
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