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Homework Help: Struggling with second part of question

  1. Jan 16, 2009 #1


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    1. The problem statement, all variables and given/known data

    Here's the entire question. But I ONLY NEED help with part B. Part A and Part C I know how to do:

    Vector A is 3.60 m with an angle of 70 degrees and Vector B is 2.4 m with an angle of 30 degrees downward.

    a) Write Vector A and Vector B in terms of the unit i and j (already completed)

    b) Use unit vectors express vector C where vector C = 3.00A-4.00B

    c) Find the magnitude and direction of Vector C

    2. Relevant equations
    Vector addition..

    3. The attempt at a solution

    Here's the solution for part A.

    Vector A= 3.60sin70=3.38
    so Vector A in terms of i and j= 1.23i + 3.38j

    Vector B= 2.4sin30=1.2
    so vector B in terms of i and j= -1.2i + -2.1j (negative since going down)

    For part b, I did trial and error, however when I looked at the answer in the back of the book, it was completely wrong. The answer is 12.01i + 14.94j and the numbers I got is much lower. I cannot do part C without getting part B first. Help!
    Last edited: Jan 16, 2009
  2. jcsd
  3. Jan 16, 2009 #2


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    Homework Helper

    It says C = 3A - 4B
    Replace A with your answer (1.23i + 3.38j) and B with your (-1.2i - 2.1j).
    Expand and simplify. The final answer should be a number times i plus a number times j.
  4. Jan 17, 2009 #3
    If angles are measured positive counter-clockwise from the positive x axis (as standard) then your value for B is incorrect.
    Last edited: Jan 17, 2009
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