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Three vectors A,B,C

  1. Aug 7, 2009 #1
    1. The problem statement, all variables and given/known data

    Three vectors, A, B, and C each have a magnitude of 74 units. Their directions relative to the positive direction of the x-axis are 15, 115, & 215 degrees, respectively. Calculate the magnitude and direction of the vectors

    a. A+B+C
    b. A+B-C
    c. C-2A


    2. Relevant equations

    R = (square root) Rx2 + Ry2


    3. The attempt at a solution

    So I started with a. :

    Ax = (74u) (cos15) = 71.478
    Ay = (74u) (sin15) = 19.152

    Bx = (-74u) (cos110) = 25.309
    By = (74u) (sin110) = 69.537

    Cx = (-74u) (cos225) = 52.325
    Cy = (-74u) (sin225) = 52.325

    Ax+Bx+Cx = 149.112
    Ay+By+Cy = 141.014

    R = (square root) (149)2+ (141)2
    R = 205 units

    Which doesnt make much sense looking at the first graph? So I attached a second graph because I am not sure which one to use.
    Any genius suggestions :smile:?

    ps. Red = A Blue = B Green = C Yellow = connected points
     

    Attached Files:

    Last edited: Aug 7, 2009
  2. jcsd
  3. Aug 7, 2009 #2

    Chi Meson

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    Homework Helper

    You stuck negatives in front of the magnitudes for vectors B and C. Why?

    The x and y components for B should be - and + respectively. Both components for C should be negative. You allow the sin and cos functions of the absolute angle to determine the pos and neg of the components.
     
  4. Aug 7, 2009 #3
    Thanks for the reply.
    I added minuses because I placed the vectors wrongly on the graph. I guess my 2nd graph was correct then.

    2nd try:

    Ax = (74u) (cos15) = 71.478
    Ay = (74u) (sin15) = 19.152

    Bx = (74u) (cos110) = -25.309
    By = (74u) (sin110) = 69.537

    Cx = (74u) (cos225) = -52.325
    Cy = (74u) (sin225) = -52.325

    Ax+Bx+Cx = -6.156
    Ay+By+Cy = 36.364

    R = (square root) (-6.156)2+ (36.364)2
    R = 36.88 units or 37 units

    ------

    How would I go forth with b. ?

    A + B - C

    Do I simply do the following:

    Ax+Bx - Cx = 98.494
    Ay+By - Cy = 141.014

    R = (square root) (98.494)2+ (141.014)2
    R = 172 units
     
  5. Aug 7, 2009 #4

    Chi Meson

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    Homework Helper

    This is apparently correct (I didn't recalculate your numbers, but they appear to be about right). Subtracting a vector is the same thing as adding a vector, except the arrow points in the opposite direction, so all components will switch signs.

    Don't forget to find the angle of direction for the resultant. I always take the arctan of (y/x) which will always be the angle from the nearest x-axis. You then have to visually determine which quadrant it lies in, and adjust accordingly for absolute angles.
     
  6. Aug 7, 2009 #5
    I almost forgot to calculate the angles. Thanks a lot!
     
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