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Math Help - vectors help?

  1. #1
    Newbie darkfenix21's Avatar
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    vectors help?

    Hey first off I hope this is the right section topost in.

    Anyways this is the question I was given I'm having trouble figurring out where I am going wrong on part c) . "In his rowboat, Pierre heads directly across a river at a speed of 10Km/hr. The river is flowing at 6 Km/hr."

    a) what is the resultant speed of the river

    answer=11.6km/hr

    b) What angle will the resultant path of the boat make with the shoreline?

    answer= 59degrees

    c) if the river is 120 meters wide how far downstream will pierre land on the opposite side?

    answer = 72meters (this is from the back of the book)

    I am unsure how they got this I tried using sine law since I have all 3 angles and a given side but I keep getting 199 meters as my answer.

    If anyone could help it really be appreciated
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  2. #2
    MHF Contributor
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    1) The resultant speed of the river?

    2) Which angle is 59.036 degrees? More specifically, is tan(59.036) = river/downstream or is tan(59.036) = downstream/river

    3) Logically, the rowing is faster than the river. The correct result cannot be greater than 120m. Rough estimate? 120 * 6/10 = 72ish without any trig. It may be exact in this case, but there is not much need to count on that, since you should be learning some trigonometry.
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  3. #3
    Newbie darkfenix21's Avatar
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    ahh makes more sense now using the 59degrees i got the angle of 31 and 120 tan 31degrees is 72.1

    thank you very much
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