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January 14th, 2008, 04:17 AM
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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? |
January 14th, 2008, 04:26 AM
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Quote:
Originally Posted by nugiboy  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.
__________________ There are two things you should never try to prove: the impossible and the obvious. The greater danger for most of us lies not in setting our aim too high and falling short; but in setting our aim too low and achieving our mark. (Michelangelo Buonarroti)
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January 14th, 2008, 04:43 AM
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Quote:
Originally Posted by mr fantastic 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 ? |
January 14th, 2008, 05:11 AM
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Quote:
Originally Posted by nugiboy 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  ,
where  is the component of the weight force (W = 117.6) parallel to the slope.
__________________ There are two things you should never try to prove: the impossible and the obvious. The greater danger for most of us lies not in setting our aim too high and falling short; but in setting our aim too low and achieving our mark. (Michelangelo Buonarroti)
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