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Inclined plane question

  1. Jan 21, 2008 #1
    So I was watching some of those physics videos posted in schoolwork board, and although he probably didn't make an error here, I don't think I understand it.

    Here's the pictues. I'll elaborate after it so you guys know what I'm talking about:

    [​IMG]


    When he broke the downward mg force into its components (mg cos theta = ma sub x and mg sin theta = ma sub y), shouldn't it be mg sin theta instead, and likewise for the other? I thought that when you break a vector into its parts, sine is associated with the y axis and cosine for the x.

    Maybe I'm having a brain fart, but I swear I had those vector components figured out.

    Thanks ahead of time.
     
  2. jcsd
  3. Jan 21, 2008 #2
    No. Draw a picture and work out the geometery. Your understanding of sin and cos is fundamentally wrong.

    see "Right triangle definitions"
    http://en.wikipedia.org/wiki/Trigonometric_function

    Hopefully, you can work that out for yourself given the link.
     
  4. Jan 21, 2008 #3

    jtbell

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    Staff: Mentor

    It depends on whether the angle is measured from the x-axis or from the y-axis. If the angle is measured from the x-axis, what you wrote is correct. If the angle is measured from the y-axis, it's the other way around.
     
  5. Jan 21, 2008 #4
    THAT'S the reason!

    Thank you, that completely did not occur to me. I understand now, thanks.
     
  6. Jan 21, 2008 #5
    One of the easiest (not fastest) ways to avoid this difficulty is to always use standard angle measurement (where the + x-axis is 0). Then x is always associated with cos and y is always associated with sin and the +s and -s take care of themselves.
     
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