Craig1998
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Hi, I'm not entirely sure how to do forces at an angle.
E.g. with this sort of queston http://img.prntscr.com/img?url=http:...om/xdmN4K1.png

How do I know whether to do sine or cosine, and how do I work out the sine and cosine of an angle when given the tangent of that angle?
My teacher's attempted to explain it using the fact that sine is for slope and that if an angle is enclosed then cosine should be used.
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Kvothe the Arcane
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(Original post by Craig1998)
How do I work out the sine and cosine of an angle when given the tangent of that angle?
You can use the fact that tan \theta=\dfrac{opp}{adj} and Pythagoras' theorem to construct a RA triangle to get your sin and cos values.


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Kvothe the Arcane
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(Original post by Craig1998)
Hi, I'm not entirely sure how to do forces at an angle.
E.g. with this sort of queston http://img.prntscr.com/img?url=http:...om/xdmN4K1.png
Have you had a look at your chapter on slopes? There should be detailed diagrams on the matter .
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Craig1998
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(Original post by keromedic)
You can use the fact that tan \theta=\dfrac{opp}{adj} and Pythagoras' theorem to construct a RA triangle to get your sin and cos values.


Say I get tan (x) = 3/5, how does this give me sin (x) and cos (x)? I don't understand trigonometry too well.
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Kvothe the Arcane
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(Original post by Craig1998)
Say I get tan (x) = 3/5, how does this give me sin (x) and cos (x)? I don't understand trigonometry too well.
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As tan(x)=3/5=opp/adj. You can construct a RA triangle for reference with the opposite side to the angle being 3 and the side adjacent to the angle being 5.

From this, you should be able to work out the missing side's length using the formula a^2=b^2+c^2 and finding out the values for sin(x) and cos(x) using the identities I provided above.
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dent1hope
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Our teacher explained it to us by saying cos is pointing downwards as it is crushing the angle and sine is for standing up (down the slope)
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Craig1998
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So if I resolved diagonally top left with the above example.
R (\): R - 3g cos(30) + 36 sin(30).

And diagonally top right for part b, then using F= mew x r.

R(/): -F + 36 cos(30) - 3g sin(30)


Just a query, say I did it both ways in the exam as I wasn't sure, and they mark whichever way scored higher (I think), would I still get the marks? The only one I was really sure about with that example was the 36 cos/sin (30).
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