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Homework Help: Subtracting/multiply vector clarification on worked problem

  1. Aug 26, 2010 #1
    Hello! I've been studying for physics and come across a problem that I've been working on from the textbook. I would just like some clarification as we have not fully covered this subject yet in the class sessions.

    1. The problem statement, all variables and given/known data
    A vector, Avector, has a magnitude of 15 w/ an angle of 25deg with x-axis, vector B has a length of 25 and makes an angle of 70 deg with x-axis. Compute the magnitudes and directions of the vectors: Cvector = Avector + 4*Bvector and Cvector = Avector - Bvector

    2. Relevant equations
    Problem 1: Cx = Ax - Bx and Cy = Ay - By

    Problem 2: Bx = KAx and By = KAy

    3. The attempt at a solution

    For the 1st problem:

    Ax = 13.6
    Bx= 8.6
    Ay = 6.3
    By = 23.5
    Leads to Cx = 5, Cy = -17.2
    Size of Cvector = 17.9 from Pythagorean theorem
    Angle of cvector = arctan (Cy/Cx) = -73.8

    For the 2nd problem:

    Bx = KAx = 4(13.6)= 54.4
    By = KAy = 4(6.3) = 25.2

    Now question here, since I have the multiplied 4B components, do I just treat the rest of the problem as an addition equation? So:

    Cx = Ax + Bx and Cy = Ay + By

    Cx = 13.6 + 54.4 = 68
    Cy = 6.3 + 25.2 = 31.5

    thank you for any help :)
  2. jcsd
  3. Aug 26, 2010 #2


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    Gold Member

    Looks OK.
    Why are you using the components of A? If B = (8.6, 23.5), then KB = (K*8.6, K*23.5).

    Yes you do, but you need to find the correct vector for 4B.
  4. Aug 26, 2010 #3
    Oh wow, can't believe I didn't notice that beforehand.

    I overlooked it because I was reading the textbook and saw the formula for multiple vectors of Bx = KAx.

    Thanks for the help, greatly appreciate it!:)
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