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Math Help - Basic vectors help ...

  1. #1
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    Basic vectors help ...

    I need to find the resultant vector c in magnitude and direction.

    I found the magnitude to be 7.2 using cosine rule, i.e.:

    c^2 = a^2 + b^2 - 2ab cos C

    However, I'm not able to find the magnitude.

    So far, I have tried the following:

    Since resultant vector c = a + b

    ax = -(6 cos 40)
    ay = 6

    bx = 3 cos 70
    by = 3

    Then,
    cx = ax + bx
    cy = ay + by

    But that doesn't seem to help.
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  2. #2
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    Quote Originally Posted by struck View Post
    I need to find the resultant vector c in magnitude and direction.

    I found the magnitude to be 7.2 using cosine rule, i.e.: Unfortunately wrong

    c^2 = a^2 + b^2 - 2ab cos C

    So far, I have tried the following:

    Since resultant vector c = a + b Correct

    ...
    I've modified your drawing a little bit.

    I placed the origin at the beginning of vector \vec a. Using the indicated angle you'll get:

    \vec a = 6 \cdot \left(\begin{array}{c}\cos(140^\circ) \\ \sin(140^\circ)\end{array} \right) ....... and ....... \vec b = 3 \cdot \left(\begin{array}{c}\cos(70^\circ) \\ \sin(70^\circ)\end{array} \right)

    Now the next steps are quite easy.

    I've got |\vec c| \approx 7.57
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  3. #3
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    Edit: errr.. never mind.

    I actually got 7.6, but wrote 7.2 in my first post here.. So I was correct at least about that... Thanks
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