1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: Vectors problem

  1. Jun 5, 2012 #1
    1. The problem statement, all variables and given/known data

    The following are all vectors:
    A = <2, 1, 1>
    B = <1, -2, 2>
    C = <3, -4, 2>

    Find the projection of (A + C) in the direction of B

    2. Relevant equations

    Dot product?

    3. The attempt at a solution

    I was not sure what the meant in this question.

    I added A and C and I got (A+C) = <5, -3, 3>

    Then I did (A+C)dot(B) and I got that equal to 16

    I was also thinking of dotting (A+C) with the unit vector of B?
  2. jcsd
  3. Jun 5, 2012 #2
    Dot product is exactly what you want to do. :smile:

    [tex]\vec{X}\cdot\vec{Y} = |X|\cdot|Y|cos\theta[/tex]

    You have the dot product, |X|, |Y|, and you need cosine of the angle :wink:
  4. Jun 5, 2012 #3

    So is the question basically asking "What is the angle between (A+C) and B?"
  5. Jun 5, 2012 #4
    No, it is asking you for the projection(component) of A+C on B. What is the component of a vector X on another vector Y when the angle between them is θ??
  6. Jun 5, 2012 #5
    ok, I see. Thanks for the help.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook