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Old Mar 19th 2015, 01:19 PM   #1
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Statics - Very basic question

I'm currently cramming for a statics final, and it seems that I understand all of the concepts but lack the fundamental geometry skills to tie it all together.

In a basic problem such as this:
http://s1303.photobucket.com/user/Tr...6xyy9.png.html

The goal is to find the the resultant forces. I get the plan: divide each force into 3 components and figure out the magnitude in each direction, then figure out the resultant.

I first found F1Z, which is simply F1sin(60).

Then I found the random line between F1 and the Y axis with F1cos(60) and denoted it as F'. This is where I was halted.

I'm looking at the solution, and the next step they used -F'cos(45) to find F1 in the X direction. Looking at the picture, it seems -F'cos(45) would give you a force along the Y axis, as cosine finds the adjacent side. What am I missing here?

Also, as a side note, what exactly is F'? I've been using it to this point without actually understanding what it is in the grand scheme of things.
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Old Mar 20th 2015, 12:09 PM   #2
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I don't see F' on the figure, but it seems to be the projection of vector F1 onto the x-y plane. It's not a random vector at all - you need it in order to calculate the next step which is the projection of f' onto the x- and y- axes. You are correct that the component of F1 in the x direction should be -F'sin(45). I suppose since sin(45) = cos(45) it doesn't affect the end result, but their solution is not right.
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