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Homework Help: Finding the sum vector given only the moduli and angle

  1. Jan 25, 2014 #1
    1. The problem statement, all variables and given/known data
    Given two 2-dimensional vectors [itex]\overline{a}[/itex] and [itex]\overline{b}[/itex] of moduli l[itex]\overline{a}[/itex]l = 3u and l[itex]\overline{b}[/itex]l = 4u, and forming an
    angle  of 120 degrees between them, determine the modulus of the sum vector [itex]\overline{s}[/itex] = [itex]\overline{a}[/itex] + [itex]\overline{b}[/itex]
    and the angle between [itex]\overline{s}[/itex] and [itex]\overline{a}[/itex].

    2. Relevant equations

    a.b = l[itex]\overline{a}[/itex]ll[itex]\overline{b}[/itex]lcosθ

    3. The attempt at a solution

    All if have calculated is the dot product of vectors a and b, coming to (6u2). I cannot seem to figure out what I can do to find the sum of these vectors only given the moduli and angle between them. Please Help :-)
  2. jcsd
  3. Jan 25, 2014 #2
    I think I have solved my own question. I think I have to use the cosine rule and not the vector product rule as stated above
  4. Jan 25, 2014 #3


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