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Can Someone Explain This Trig Picture to Me?

  1. Sep 14, 2013 #1

    Meadman23

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    Its the attached picture. I'm not seeing why when using the dot product, they start using sin.
     

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    Last edited: Sep 14, 2013
    Meadman23, Sep 14, 2013
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  3. Sep 14, 2013 #2

    SteamKing

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    The picture is self explanatory. If you know trig, it should be elementary.
     
    SteamKing, Sep 14, 2013
  4. Sep 14, 2013 #3

    UltrafastPED

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    Projections onto different axes.
     
    UltrafastPED, Sep 14, 2013
  5. Sep 15, 2013 #4

    Meadman23

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    I'm just not getting it. '
    I see that the angle between r and x is phi, thus in using the formula, r (dot) x = (1)(1) cos(phi).

    Then I see the angle between r and y is (90-phi), thus in using the formula, r (dot) y = (1)(1)cos(90-phi) = sin (phi).

    I then see the angle between phi and x is (90 +phi), thus in using the formula, phi (dot) x = (1)(1)cos(90+phi) = -sin (phi)

    I then see the angle between phi and y is (180 - phi), thus in using the formula, phi (dot) y = (1)(1)cos(180-phi) = -cos (phi)???

    I don't get why the last one is +cos(phi)....
     
    Meadman23, Sep 15, 2013
  6. Sep 15, 2013 #5

    UltrafastPED

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    Look at the blue part of the diagram ... phi(dot)y = cos(phi).
     
    UltrafastPED, Sep 15, 2013
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