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Homework Help: Crossing a river

  1. Oct 11, 2009 #1
    1. The problem statement, all variables and given/known data
    A swimmer wants to cross a river, from point A to point B, as shown in the figure. The distance d_1 (from A to C) is 200m , the distance d_2 (from C to B) is 150m, and the speed v_r of the current in the river is 5 km/h. Suppose that the swimmer's velocity relative to the water makes an angle of [tex]\theta = 45 degrees[/tex] with the line from A to C, as indicated in the figure.

    To swim directly from A to B, what speed u_s, relative to the water, should the swimmer have?

    2. Relevant equations
    I would think that you would use nothing more than some trig, Pythagorean theorem and stuff about adding and subtracting vectors

    3. The attempt at a solution

    I have no idea how to attempt this solution. I don't know where to even start. I know the answer is 4.04km/h but I have no idea how to get to that answer. Where should I start?

    Attached Files:

  2. jcsd
  3. Oct 11, 2009 #2
    Well don't all jump up at once now!
  4. Oct 12, 2009 #3

    Doc Al

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    Staff: Mentor

    What direction must the swimmer's velocity have with respect to the shore? Hint: The velocity of the swimmer with respect to the shore = velocity of the swimmer with respect to the water + velocity of water with respect to the shore.
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