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Homework Help: Relative motion problem

  1. Sep 27, 2004 #1
    A boat takes 3.20 hours to travel 10.0 km down a river, then 6.10 hours to return. How fast is the river flowing?

    can someone give some suggestions as to how to do this? im guessing you use reference frames. thanks in advance.
  2. jcsd
  3. Sep 27, 2004 #2
    Use the data you have to find the velocity each way, then you should be able to finish it from there.
  4. Sep 27, 2004 #3
    what am i supposed to do with the velocities? i tried taking the difference of the velocities divided by the difference in time but that was wrong. am i supposed to find the velocity of the boat itself and find it relative to the actual rate the boat traveled?
  5. Sep 27, 2004 #4
    Remember that when the boat is travelling with the current, the velocity of the boat relative to the water plus the velocity of the water equal its total velocity relative to the shore (you were right, this is about reference frames). On the return trip...you guessed it, the velocities are subtracted. Can you set up an equation with one variable to solve for?
  6. Sep 27, 2004 #5
    ahh yes i got it now
    basically set up two equations vb + vw = downstream velocity and vb - vw = upstream velocity. find vb (velocity of boat) by adding both given velocities and divide by two. then solve for vw (velocity of water) using either of the equations.
  7. Sep 28, 2004 #6
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