decompose a force to its components

hi, i am rreading some examples in statics and as i decompose the force into its components i have notices that although (for example) the angle between the force and the x-axis is 45 degrees when we work the example we wometimes take angle of 135 degrees. I understand that it is 90+45 but my question is....do we start counting the degrees from the positive x-axis or in general from the positive x,y,z axis and we move clockwise ?
Any input would be appreciated.
c_hocklandAsked:
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Carl BohmanCommented:
Degrees are normally measured based on the positive axis and a counter-clockwise direction.   You would use 135 degrees is the force is 45 degrees "up" (in the y-direction) from the negative x-axis (which means it is positioned in quadrant 2 of the x-y plane).

Quadrants:
 
    |
 II | I
----+----
 III| IV
    |

Open in new window

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c_hocklandAuthor Commented:
hi , thanks for the quick feedback. I am attching a screenshot from one of the examples. Why did he come up with the cos135 degrees ? since the angle is 45 degrees ? ( see screenshot with my comment-question there)

many thanks.
Capture-07082009-123753.BMP
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Carl BohmanCommented:
The convention is normally to have the x point right, y point up, and z point towards you (out of the page). There is no requirement to follow this convention, however, and this graph shows x where z normally is, y where x normally is, and z where y normally is.
The vector is projecting onto the x-y plane (the "floor"/"bottom" of this graph), but the projection is ending up behind the positive y axis.  This is quadrant 2 on the x-y plane.  Thus, 135 degrees.
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c_hocklandAuthor Commented:
thnaks for the great input. I will go back and "study" some more the graphs so as to "get" the concept and be able to visualize easier.
again , thanks much.
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