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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:


    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"

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


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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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