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Homework Help: Calculate angles from axis with a 3d vector

  1. Jun 10, 2009 #1
    1. The problem statement, all variables and given/known data

    A vector is given by R(vector) = 1.90 ihat + 1.30 jhat + 2.92 khat.

    Find the magnitude of the vector

    Find the angle between the vector and the x, y, and z axis

    2. Relevant equations

    I have read something about dot product but im not sure if it applies here

    3. The attempt at a solution


    R(magnitude in xy plane)=2.302

    R(magnitude in yz plane)=3.196

    R(magnitude in xz plane)=3.484

    I dont know if I should have solved for these or if i should try to solve for the angles in each plane specifically. I was just getting numbers to see where it got me.
  2. jcsd
  3. Jun 10, 2009 #2


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    Homework Helper

    Remember that for dot products
    \vec{u}\cdot\vec{v} = |u||v|\cos{\theta}[/tex]

    Where theta is the angle between the vectors. So choose u = your vector R, and v = ihat, and you get the angle between R and the x-axis.
  4. Jun 10, 2009 #3
    arent v(vector) and v(magnitude the same which follows for u. which means that it would just be cos( 0 ) b/c that is one
  5. Jun 10, 2009 #4


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    Are you asking if [itex]\vec{v}[/itex] and [itex]|v|[/itex] (to use nicksauce's notation) are the same? They're not... [itex]\vec{v}[/itex] is a vector, while [itex]|v|[/itex] is a scalar. Completely different things. It's like the difference between an arrow and the length of the arrow.

    Do you know how to take the dot product of two vectors?
  6. Jun 10, 2009 #5
    no thats what im basically asking, we were not taught this but need to know it i guess to do this problem. Also it is not anywhere in the chapter's we are studying
  7. Jun 10, 2009 #6
    I have to figure this out by tomorrow and i cant really figure out how to do it from looking up up anywhere. Can someone please resolve this one before it is due tomorrow?
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