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Homework Help: Physics Kinematics Question.

  1. Aug 2, 2010 #1
    1. The problem statement, all variables and given/known data

    A motorboat going downstream overcame a raft at a point A;
    • = 60 min later it turned back and after some time passed the raft
    at a distance l = 6.0 km from the point A. Find the flow velocity
    assuming the duty of the engine to be constant.



    2. Relevant equations

    ---------------------------------

    3. The attempt at a solution

    I assumed the stream velocity as v0 and the velocity of the motorboat with respect to water as v' . The motorboat reached point B while going downstream with velocity (v0 + v') and then returned with velocity (v' - v0) and passed the raft at point C . Then I assumed t to be the time for the raft to move from point A to C , during which the motorboatboat moves from A to B and then from B to C .

    But after this I have no idea how to proceed .
    Pls Help!
     
  2. jcsd
  3. Aug 2, 2010 #2
    Let [tex]v_{0}[/tex] be the velocity of the stream, [tex]v[/tex] velocity of the motorboat, [tex]t_{0}[/tex] time after the motorboat changes direction (60 minutes) and [tex]L[/tex] distance form point A (6 km).

    After 60 minutes, distance between boats is

    [tex]d=vt_{0}.[/tex]

    At that moment, motorboat is at point B. After time [tex]t[/tex] passes, the boats meet at point C. So, they travel their own distances in the same time interval [tex]t[/tex]. Therefore

    [tex]d=vt_{0}=v_{0}t+(v-v_{0})t.[/tex]

    And we also know

    [tex]L=v_{0}(t+t_{0}).[/tex]

    [tex]v_{0}[/tex] is easy to obtain from those equations.
     
  4. Aug 5, 2010 #3
    But the motorboat is only an observer, otherwise irrelevant. The raft travelled 6 km in 60 minutes.
     
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