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Question on vectors ?

  1. Feb 28, 2008 #1
    1. The problem statement, all variables and given/known data
    A boat needs to sail due north , but the current is flowing southeast at a speed of 20km/hr. In still wate, the boat has a maximum speed of25km/hr .

    (a) In what direction will the boat actually move (relative to an observer on land)?

    (b) What's the course (direction) necessary to that ship could travel North ?

    (c) What is the speed of the boat relative to an observer on land?

    2. Relevant equations


    3. The attempt at a solution

    (a) North .

    (b) East . I using vector theory .

    (c) Using theorem phythogoras :

    Speed^2 = 25^2 - 20^2
    = 225
    speed = 15 km/hr

    Am I correct ? PLease give me the answer .
  2. jcsd
  3. Feb 28, 2008 #2


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    Homework Helper


    As you know, Pythagoras' theorem only works for right-angled triangles, and you don't have a right-angled triangle here.

    You only know two directions and two lengths and the angle between them. That gives you a triangle for (a) and another one for (b).

    Draw the triangles (only roughly - I'm not suggesting you measure them!), to make sure you've got them the right way round.

    Then use sines or cosines to work out the unknown length and the unknown angle! :smile:
  4. Feb 28, 2008 #3


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    Staff Emeritus
    Science Advisor

    Remember that the "sine law" and "cosine law" let you use sine and cosine on non-right triangles. As tiny-tim said, start by drawing a picture and identifying the parts of the triangle.
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