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Homework Help: The sum of two vectors given magnitudes and included angle

  1. Jul 25, 2012 #1
    1. The problem statement, all variables and given/known data

    Given the magnitudes of vectors u and v and the angle θ between them, find sum of u + v. Give the magnitude to the nearest tenth when necessary and give the direction by specifying the angle that the resultant makes with u to the nearest degree.

    2. Relevant equations

    |u| = 15, |v| = 15, θ = 116°

    3. The attempt at a solution

    Knowing only the answer (15.9, 58°) and some trig ideas:

    I draw an angle of 116 degrees in the starting point of the trig plane. I drop a line from the angle end side, forming a triangle with a 64 degree angle in quadrant II. The other angles of the triangle are both 58 degrees (116 deg / 2). Both opposite (U) and adjacent (V) sides are 15.

    SAS - law of cosines

    c^2 = (15)^2 + (15)^2 - 2 (15)(15) cos (64 deg)

    c = 15.9

    angle = 58 deg

    (15.9, 58 deg)

    What is the correct method?
  2. jcsd
  3. Jul 25, 2012 #2


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    I think that is the correct method. Well done! Draw the quadrilateral that the two vectors and the sum make and use trig.
  4. Jul 25, 2012 #3
    Can you verify this?

    |u| = 6, |v| = 11, θ = 42

    I drew the quadrilateral. From this, 138 deg captures (u + v). Law of cosines using 6, 11, and 138 deg. c = u + v = 15.97. Round to 16. 42 bisected = 21. Calculated answer (16, 21 deg), however, the answer key presents (16, 27 deg). I have no idea how that angle can be 27 so that must be a mistake.
  5. Jul 25, 2012 #4


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    In your last example you had |u|=|v| so you could just bisect the angle. Here |u| and |v| are different. So the angle of u+v won't bisect the angle. You'll have to do a little more trig to find the right angle. You'll want to find the angle between the sides that are 6 and 16 in the triangle whose sides are 6, 11 and 16.
  6. Jul 26, 2012 #5
    Got it, thanks. The angles of the 6-11-16 triangle are 138 deg, 15 deg, and 27 deg. I used the law of cosines. 27 deg is the angle between u and u + v.
  7. Jul 26, 2012 #6
    As written there are an infinite number of answers since u + v is a vector sum. Are you sure it isn't |u + v|?
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