Posted: 10/3/2005 7:19:36 PM EDT
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Ok, im sort of stuck on this. Havent done algebra stuff for awhile and i cant solve this equation. 230=360sin(theta) - 63cos(theta) trying to get theta Thanks |
I SUCK at math
Im going for civil. I tried using cos(theta) = (1-(sin^2(theta))^.5 but my value for theta doesnt work in the original equation then |
let me get back to you on that.... 
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sin and cos are functions of theta, so ive got to keep them together |
Just say no. 
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you should see how the problem started - A force P is applied to the lever of an arbor press. Knowing that P lies in a plane parallel to the yz plane, and Mx= 230 lb in, My= -200 lb in, and Mz= -35 lb in, determine the magnitude of P and the values of theta and phi. |
| yeah will that work? square everything and insert (1-cos theta squared) for sin squared, multiply all that out, and then take the square root of the whole shebang and take the inverse sin to get theta? I bet that's it..that damn pathagoriem thereom or something |
for some dumb reason i keep getting something that doesnt work, I must be screwing up when i simplify the radical |
I am going to say that you set the problem up wrong. |
Dude, VALUABLE INFO. I'm thinking you'r math teacheer is mean throwing in sin and cos. If you got a problem, state the WHOLE problem. Don't leave out details! 
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Here is how to solve it without remembering how to do the math. Do a trial-and-error solution. Use a spreadsheet like exel. But first, spread sheets like angles in radians not degrees, so you have to change the angles from degrees to radians, here is how. (radians = degrees/360 * 2 * 3.1416). Use a more accurate version of PI though. Next, set up Place for a staring angle, like zero degrees and a place for increment size, like one. That is, say, the starting degrees in A5 and the starting increment in B5. Then build a column for the degrees that starts with the 'starting degrees' and increments with the 'increment size'. For instance start in A10 by using =A5. In A11 you would have =A10+$B$5. Copy this A11 cell down the page until it gets to 360 or 400. Next, build a column for theta. I would start in C10 with =(360*sin(A10/360*2*3.1416)-63*cos(A10/360*2*3.1416)). Copy this cell down the C column, as far as the A column goes. But, use the PI built into the machine or a more accurate PI than I used. Look for something closest to 230 in the C column. You will see the answer lies after 40 and before 50 degrees. Then change the 'start degrees' to 40 and the 'increment size to 0.1. Hmmm, we need to change the 'start degrees' to more like 48. OK, change the increment size to 0.01. Keep changing the 'start degrees' and 'increment size' until you "converge on a solution". Take the A and C columns out to four places when you are almost "converged". I got: theta = 48.9265, giving 230.000. But a longer version of PI would have given a slightly more accurate answer. In computing, this sort of trial and error solution is called a newton-raphson solution, aka numerical solution. This "numerical solution" technique is used in engineering a lot, because only four percent of all differential equations have a math-department solution (analytical solution), but engineers still have to solve those equations. I hope this helps. Mike S |