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Confusion regarding cos or sin

  1. Sep 3, 2015 #1
    1. The problem statement, all variables and given/known data
    I have the solution and everything, I'm just confused why they use cos for Fy and sin for Fx ...
    2. Relevant equations
    I've always known Fx=Fcos and Fy=Fsin .. but now im getting to different problems and it confuses me
    3. The attempt at a solution
    I read many posts and answers such as "take sin as opposite to theta" but I cant understand... I will apreciate if you could easily put this into brain! check image for example , if you got videos that help please share i have an exam tomorroW
     

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  2. jcsd
  3. Sep 3, 2015 #2

    mfb

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    It depends on the way you define the angle.

    For the side that is opposite to the angle, you need the sine, for the side next to the angle you need the cosine. That's just the definition of the two (more precise: one possible definition). No logic behind it, just one of the few things you have to learn by heart.
     
  4. Sep 3, 2015 #3

    can you explain your concept using the attached image pls?
     
  5. Sep 3, 2015 #4

    SteamKing

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    This is the image you should memorize:

    http://demo.activemath.org/ActiveMath2/LeAM_calculusPics/TrigFunctionsTriangle.png?lang=en [Broken] ​
     
    Last edited by a moderator: May 7, 2017
  6. Sep 3, 2015 #5
    I know this and memorized this, even using SOH CAH TOA,, but I cant apply it to the problems im getting now
     
  7. Sep 3, 2015 #6

    SteamKing

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    Well, why don't you take F1 in Prob. 2-145 and identify the parts of the force triangle? IOW, what are the components of F1 ?
     
  8. Sep 3, 2015 #7
    well using the concept of "use sine for side opposite to angle" .. then Id say Fx=30sin30 and also Fy=30sin30 ... becausee I take the side and it turns out to be opposite to the angle... this is how I'd solve it and its wrong anyways
     
  9. Sep 3, 2015 #8

    SteamKing

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    I can't make out your justification here.

    Clearly, the diagram for F1 shows that the angle is 30°. The hypotenuse of the triangle is the line of action with the arrow at the end. There are only two other sides to the triangle, and there can be only one side which is opposite of the 30° angle and one side which is adjacent.

    Hint: the opposite side to the angle is the side which doesn't form the angle. The adjacent side is that side (not the hypotenuse) which does form the angle.
     
  10. Sep 4, 2015 #9

    CWatters

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    If you are familiar with the drawing Steamking posted then just turn it around until the arrangement matches what's on the problem statement. For example..

    Angles.png
     
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