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Homework Help: Vector Question

  1. Feb 3, 2007 #1


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    Gold Member

    1. The problem statement, all variables and given/known data
    A Family has a boat which can travel at 12 m/s in still water. They are using it to reach a point 500m downstream on the other side of a river 300m wide, and flowing at 4 m/s. What heading must they take, relative to their starting point, in order to reach their destination?

    3. The attempt at a solution
    I first drew a triangle, with the side parallel to the river banks 500m, and the side perperndicular 300m. From there, i found that to end up 500m downstream, the resultant direction of travel should be ~59 degrees from the perpendicular (or 31 degrees from the bank).

    The rivers current plus the boats motion should result in a velocity in the direction of 59 degrees. So:

    \left( {\begin{array}{*{20}c}
    4 \\
    0 \\
    \end{array}} \right) + \left( {\begin{array}{*{20}c}
    {12\sin \theta } \\
    {12\cos \theta } \\
    \end{array}} \right) = \lambda \left( {\begin{array}{*{20}c}
    {100\sqrt {34} \sin 59} \\
    {100\sqrt {34} \cos 59} \\
    \end{array}} \right)

    I solved for theta, and found that it equals 49 degrees. So i concluded that to end up 500m downstream, the boat should set off at 49 degrees to the perpendicular of the banks.

    Im unsure if that is correct. Also, is there a better way i should have gone about it?

  2. jcsd
  3. Feb 3, 2007 #2

    Gib Z

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    Homework Helper

    Are the matrices really necessary? Other than that, that seems like a good way to do it. You can check if its correct by yourself, add up the vectors assuming 49 degrees is correct, see if it gets you where you want.
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