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Troublesome vectors

  1. Oct 13, 2005 #1
    I have 2 vectors, which are plotted using the tail-tip method to form a non-right triangle.

    Side A is vector 1, side B is vector 2, and side C is my resultant.
    I know I can get angle C, but that's where I'm stuck.

    How do you get angle C? In simple terms....:confused:
     
  2. jcsd
  3. Oct 13, 2005 #2

    HallsofIvy

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    It's a bit confusing to talk about "side C" and "angle C"! Standard notation is that the lengths of the three sides of the triangle are a, b, c and the angles opposite each are A, B, C respectively. If you know a, b, c, then use the cosine law to find C: c2= a2[/sup]+ b2- 2ab cos(C). Plug in a, b, c and solve for C.
     
  4. Oct 13, 2005 #3
    I don't have c. I have a, b, and their degrees, but no c. I don't have A or B either.

    I'm trying to sove for C, but I must have c before I can use the law of cosines.
     
  5. Oct 13, 2005 #4
    how about law of sins

    sin A / a = sin B / b = sin C / c
     
  6. Oct 14, 2005 #5

    HallsofIvy

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    You orginally said "Side A is vector 1, side B is vector 2, and side C is my resultant" so it was reasonable to assume that you knew those 3 lengths. In vector problems, normally you know two vectors, say lengths a and b, and the angle between them, but that would be C which you say you don't know. If lengths a and b are the only values you have, you can't solve this: two parts of a triangle are not enough! What information do you have?
     
  7. Oct 14, 2005 #6
    You have to understand that the resultant vector componants are the sum of the adding vectors. Add the vertical component of the vector A to the vertical component of vector B. Then add the horizontal component of the vector A to horizontal component of vector B. Then you have the components of vector C and the magnitude of C = ((Ay + By)^2 + (Ax + Bx)^2)^1/2
     
  8. Oct 14, 2005 #7

    HallsofIvy

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    That's one way to do it. It may or may not be simpler than adding the vectors "geometrically". The question "Confused one" initially asked made it clear, I thought, that he was attempting to do this geometrically. I sure wish he would get back to us and tell us exactly what information he has to work with. I would have suggested that he find angle C by just subtracting the angles the the vectors make with some fixed line, but surely he wouldn't be having so much trouble if he were given the vectors like that.
     
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