# Axis Rotation and Vector magnitude problem not sure if answers are right

Hey guys I'm sort of new to this forum, but I heard it was good so I thought i'd post a few questions here

## Homework Statement

The first question I have is not really a problem, but more of a question on the problem itself. I have two 3d vectors which are then subtracted to form a 3d vector in the xz plane, I need to find the direction of this vector, and it makes a 3,4,5 right triangle so i know the angle is 36.9 degrees, the question though is since its in the xz, would it still be S of W if it was in the 3rd quadrant? How would I label the direction of it? Would it even be in terms of North South East and West?
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[PLAIN]http://img825.imageshack.us/img825/6527/lolphysics.png [Broken]

2) Find the magnitude and direction of the sum R of the three vectors shown above; R = A + B + c. The vectors have the following magnitudes, A = 5.0 B = 7.9 and C = 8.0, express the direction of the vector sum by specifying the angle it makes with the positive x-axis with the counterclockwise angles taken to be positive

## The Attempt at a Solution

For the second one, I split them into components

A = 5i + 0j
B = 0i - 8j
C = -3.95i + 6.84j

to find the values for C, I used X = 7.9cos120 and Y = 7.9sin120

This gives an R value of 1.05i - 1.16j = R, which gives a magnitude of 1.56 and an angle of -47.8 degrees?

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Spinnor
Gold Member
I'm comfused %^/

I'm having to struggle to understand the problem and I'm tired. I should go to bed.

Anyone?

vela
Staff Emeritus