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Math Help - Trig confusion

  1. #1
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    Trig confusion

    Hi

    Im a bit confused on working out trig equations. I know all the rules, but something doesn't make sense to me.

    There is a question in my excercise book, which i worked out and i ended up with having to solve:

    Sin 5 = 117.8 / x

    Using the normal rules of equations, i got

    x sin 5 = 117.8
    117.8 / sin 5 = x
    x = 1351

    But the book said to do:

    117.8 sin 5 = x
    x = 10.3

    Im a bit confused on why this works. Surely you multiply by the the denominator. Any explanations?
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  2. #2
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    Quote Originally Posted by nugiboy View Post
    Hi

    Im a bit confused on working out trig equations. I know all the rules, but something doesn't make sense to me.

    There is a question in my excercise book, which i worked out and i ended up with having to solve:

    Sin 5 = 117.8 / x Mr F says: Let me guess. You're using trigonometry to find the length of a side in a right-triangle. This expression you got, sin 5 = 117.8 / x, is probably wrong. It's probably meant to be sin 5 = x/117.8 .....

    Using the normal rules of equations, i got

    x sin 5 = 117.8
    117.8 / sin 5 = x
    x = 1351

    But the book said to do:

    117.8 sin 5 = x
    x = 10.3

    Im a bit confused on why this works. Surely you multiply by the the denominator. Any explanations?
    Please give the question in your exercise book - exactly as it is written.
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  3. #3
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    Quote Originally Posted by mr fantastic View Post
    Please give the question in your exercise book - exactly as it is written.
    Here it is:



    Its part iii im stuck on.

    I need to find the force on the horizontal before i can use f = ma to find the acceleration. This basicly simplifies to this:



    (Edit: the angle in this diagram should be 5' not 15')

    Sin = opp / hyp

    Therefore in this example, wouldnt sin 5 = 117.8 / hyp ?
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  4. #4
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    Quote Originally Posted by nugiboy View Post
    Here it is:



    Its part iii im stuck on.

    I need to find the force on the horizontal before i can use f = ma to find the acceleration. This basicly simplifies to this:



    (Edit: the angle in this diagram should be 5' not 15')

    Sin = opp / hyp

    Therefore in this example, wouldnt sin 5 = 117.8 / hyp ?
    No.

    Read the attachment. Pay close attention to the triangle showing the resolution of the weight force into components parallel and perpendicular to the slope - note that

    \sin 5^0 = \frac{W_{parallel}}{W} = \frac{W_{parallel}}{117.6} \Rightarrow W_{parallel} = 117.6 \sin 5^0,

    where W_{parallel} is the component of the weight force (W = 117.6) parallel to the slope.
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