1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

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.
     
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook