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: Finding Magnitude of Vector

  1. Jan 28, 2012 #1
    1. The problem statement, all variables and given/known data
    Let u = <3,-1>. Find v such that ||v||=4 and the angle between the vectors is ∏/3.

    2. Relevant equations

    u.v = ||u|| ||v|| cos θ

    3. The attempt at a solution

    <3,-1>.v = (√10)(4) cos ∏/3
    <3,-1>.v = 2√10
    v = <(2√10)/3, (2√10)/-1>

    Can you please confirm that I am doing this correctly, or if not help me through where I am going awry?
  2. jcsd
  3. Jan 28, 2012 #2


    User Avatar
    Staff Emeritus
    Science Advisor
    Homework Helper
    Gold Member

    You're not doing this correctly. The v you have has a magnitude that's way too big.

    Write vector, v, as v = <vx, vy>, where vx2 + vy2 = 16 .
    Solve this for vy.​

    Your lines:
    <3,-1>∙v = (√10)(4) cos ∏/3

    <3,-1>∙v = 2√10​
    are fine.

    But in the next line, it looks like you tried to divide by the vector <3,-1>. There is no such thing as vector division !

    Compute <3,-1>∙v = (3)vx + (-1)vy.

    Of course that gives you: (3)vx + (-1)vy = 2√(10)

    Solve that equation and the equation, vx2 + vy2 = 16, simultaneously.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook