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Homework Help: Cosine vectors equation

  1. May 12, 2008 #1


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    1. The problem statement, all variables and given/known data
    1. The problem statement, all variables and given/known data

    A river is 2 km wide and flows at 6 km/h. A motor boat that has a speed of
    20 km/h in still water heads out from one bank perpendicular to the current.
    A marina lies directly across the river on the opposite bank. Use Geometric
    Vectors to solve this problem.

    a. How far downstream from the marina will the boat reach the other bank? (Answer: 0.6 km)
    b. How long will it take? (Answer: 6 min)

    2. Relevant equations

    sine and cosine reltions

    3. The attempt at a solution

    I've found that the resultant velocity of the boat would be 20.88 km/h, but I am not sure how to find the distance (horizontal) that the boat travells.
  2. jcsd
  3. May 12, 2008 #2
    It's a triangle. Compute all the sides.

    2 km, .6 km, 2.08 km

    BTW, what makes you think the velocity of the perpendicular boat isn't 20km/h ?
  4. May 12, 2008 #3


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    Sorry, but how do you compute all three sides when you only have the perpendicular distance of 2km?
  5. May 12, 2008 #4


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    Science Advisor
    Homework Helper

    Hi a.a! :smile:

    The question does not ask you for the resultant velocity, and you don't need to find it to answer the question.

    Think again. :smile:
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