1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: Axis Rotation and Vector magnitude problem not sure if answers are right

  1. Sep 12, 2011 #1
    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

    1. The problem statement, all variables and given/known data

    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?
    [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

    2. Relevant equations

    3. 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?
    Last edited by a moderator: May 5, 2017
  2. jcsd
  3. Sep 12, 2011 #2
    I'm comfused %^/

    I'm having to struggle to understand the problem and I'm tired. I should go to bed.
  4. Sep 13, 2011 #3
  5. Sep 13, 2011 #4


    User Avatar
    Staff Emeritus
    Science Advisor
    Homework Helper
    Education Advisor

    Frankly, I wouldn't use N, S, E, and W designations, even with the xy-plane, unless you're talking about directions on the surface of the Earth. You just need to describe the direction so it makes sense to whomever is reading your work. You could draw a picture of the xz-plane with the vector and angle clearly labeled. Unlike with the xy-plane, there's no standard convention for specifying a direction as far as I know.

    In the work for the second problem, you switched B and C from what the picture says, but your method is correct. I didn't check your actual numbers. I'll assume you punched everything into the calculator correctly.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook