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Homework Help: Finding the angle in a vector

  1. Sep 16, 2007 #1
    [SOLVED] Finding the angle in a vector

    hey these are normally easy, but I keep getting this wrong
    Here are three displacements, each in meters: d1 = 5.0 i + 5.0 j -6.0 k, d2 = -1.0 i -1.0 j + 3.0 k, and d3 = 4.0 i + 3.0 j + 2.0 k.

    (a) What is r = d1 - d2 + d3?
    Got it right, 10,9,-7
    (b) What is the angle between r and the positive z axis?
    I am getting 127.87 degrees, but its wrong

    I am taking the vector r's j and k, or y and z axes. then I am just doing sqrt(9^2 + -7^2) and then cos-1(-7/11.4(which is the mag)) can anyone help this is due in like 35 mins
  2. jcsd
  3. Sep 16, 2007 #2
    Are you using the dot product, or trig? Regardless, your magnitude for r is wrong, it should be sqrt(9^2 + 7^2 + 10^2). So the z leg is -7, which you got, and the magnitude gives the new magnitude. The angle would just he inverse cosine of that proportion.
  4. Sep 16, 2007 #3
    if I put the mag at 15.17, it would mess it up right, because the vectors r axis doesn't have anything to do with z right, its up 9 and out -7 if you look at it straight on

    are you saying its 117.49?
  5. Sep 16, 2007 #4
    when finding the magnitude of a vector you can't just ignore one of the components.

    0i+9j-7k =/= 10i+9j-7k

    so the magnitude of the just 9j-7k is not equal to the magnitude of 10i+9j-7k
  6. Sep 16, 2007 #5
    Hmm, I'm not really sure what you mean by the r axis. Take three pencils and put them on the edge of table, each one representing the respective axis, and then point a stick out and down to represent your r vector. It wants the angle between the stick and the z axis directly, so ignore the x and the y pencils, and figure out the angle between the stick and the z pencil.

    What you are proposing is to cut up some of the stick, and move it.
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