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Homework Help: Relative Motion - River/Boat problem

  1. Jan 29, 2009 #1
    1. The problem statement, all variables and given/known data
    A boat takes 2.80 hours to travel 30.0 km down a river, then 4.50 hours to return. How fast is the river flowing

    2. Relevant equations
    Vac = Vab + Vbc

    3. The attempt at a solution
    I wasn't quite sure how to use the formula that the professor gave us which is above for this particular problem. The way I see it, since the problem doesn't say their is a change in acceleration, I'm thinking the speed of the boat IS the speed of the river seeing as it is saying "down" a river..... so I just took 30 / 2.8 which gives me the speed of the boat but.... It's not the answer.
  2. jcsd
  3. Jan 30, 2009 #2
    There are a couple of steps involved here:

    1. Set up two equations for the speed of the boat when it is going with and against the current.

    2. Set up an equation in which you state that the distance travelled in both cases in exactly the same.

    3. Find a relationship between the boat speed and the current speed using the above.

    4. Use the fact that they give you the distance travelled to your advantage. With this, you can set up another equation relating the boat and current speed. With two equations and two unknowns, you can finish the problem.
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