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Homework Help: Subtracting Vectors

  1. Sep 5, 2007 #1
    http://img523.imageshack.us/img523/4889/yf0148vj7.jpg [Broken]​
    1. The problem statement, all variables and given/known data

    Use unit vectors to express the vector C, where C = 3.00A - 4.00B

    Keep in mind that C, A, and B are vectors, I could not get the Latex to work.

    2. Relevant equations

    There are no relevant equations, but I don't know if the question above the one I am asking refers to this, as I got that:

    Vector A = 1.23 i + 3.38j
    Vector B = -2.08i - 1.2j

    3. The attempt at a solution

    My attempt would be to subtract -2.08 from 1.23 leading the final i to be 3.31 (as it's subtracting and negative with negative = +.) and the final j to be 4.58.

    If this is how it is suppose to go, then do I plug those numbers in for A and B, multiply time their coeffiecients

    ex: 3.00 (3.31) - 4.00 (4.58) = C ?

    Last edited by a moderator: May 3, 2017
  2. jcsd
  3. Sep 5, 2007 #2
    No. What you're attempting to do is to multiply the differences of the i and j components by 3 and 4, respectively. What you actually need to do is multiply the vector A by 3, multiply the vector B by 4 and then take their difference. Multiplying a vector by a scalar is equivalent to multiplying each component of the vector by that scalar.
  4. Sep 5, 2007 #3
    *EDIT* deleted solution
    You were solving for [tex]3.00(\vec{C}\hat{i}) + 4.00(\vec{C}\hat{j})[/tex]

    But it asks for [tex]3.00\vec{A} - 4.00\vec{B}[/tex] or
    [tex]3.00\vec{A}\hat{i} + 3.00\vec{A}\hat{j} - (4.00\vec{B}\hat{i} + 4.00\vec{B}\hat{j})[/tex]
    Last edited: Sep 5, 2007
  5. Sep 5, 2007 #4
    odie5533, as per the forum's guidelines, you are not supposed to reveal the solution to the problems posted in the homework help forums. At most, you can guide the person to the solution. Apart from being a rule, it is also good practice to not provide the solution, since by copying the answer the poster does not gain anything apart from a few points for a single question.
    Last edited: Sep 5, 2007
  6. Sep 5, 2007 #5
    Sorry neutrino. Fixed my response.
  7. Sep 5, 2007 #6
    @ odie5533 Don't worry about me looking at the solution, I logged in too late:rolleyes: , but I thank you for the explanation.

    [tex]3.00\vec{A}\hat{i} + 3.00\vec{A}\hat{j} - (4.00\vec{B}\hat{i} + 4.00\vec{B}\hat{j})[/tex]

    Here is what I did, I just don't know where it goes from here.

    [3.00 (1.23 i) + 3.00 ( 3.38j)] - [(4(-2.08i) + 4(-1.2j))] =

    3.69i + 10.14j + 8.32i + 4.8j

    Do I add the like terms resulting into:

    12.01i + 14.94j


    PS: I'm here to learn, as I would need to know all of this later on.:approve:
  8. Sep 5, 2007 #7
    Yes, Heat. Exactly how you have done it is right. First multiplying the components by the constant, then performing the vector subtraction.
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