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: Vector decomposition

  1. Aug 25, 2016 #1
    1. The problem statement, all variables and given/known data

    The force on an object is F = -11j. For the vector v =-i-5j, find:

    1. The component of F parallel to v
    2. The component of F perpendicular to v
    3. The work, W, done by force F through displacement v

    2. Relevant equations

    ProjvF = v dot F/ |v|2
    OrthvF = F - ProjvF
    W = D dot F

    3. The attempt at a solution

    F = < 0, -11> v = < -1, -5>

    1.
    ProjvF = -1(0) + -5(-11) / 12 + -52 = 55/26<-1, -5>
    ProjvF = <-2.115, -10.576>

    2.
    OrthvF = < 0, -11> - <-2.115, -10.576> = 0 + 2.115 , -11 +10.576 = <2.115, -0.424>
    OrthvF = <2.115, -0.424>

    3.
    W = <2.115, -0.424> dot < 0, -11> = 2.115(0) + -0.424(-11) = 4.664
    W = 4.664


    I am told that step 1 is correct, and the first value in step 2 is correct. I cannot figure out what I am missing on part 2, which is ultimately messing up step 3. Any help would be appreciated.
     
  2. jcsd
  3. Aug 25, 2016 #2

    BvU

    User Avatar
    Science Advisor
    Homework Helper
    2017 Award

    In 3 you don't do ##\vec v\cdot\vec F## but you use the one that's perpendicular.
    In 2 I don't see what's wrong.
     
  4. Aug 25, 2016 #3
    Thanks, I see what I did wrong on part 3. Part 2 after redoing it again was expecting -0.42307, I rounded too soon it seems
     
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted