1. PF Contest - Win "Conquering the Physics GRE" book! Click Here to Enter
    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!

Given vectors how to find third equation

  1. Jun 2, 2014 #1
    1. The problem statement, all variables and given/known data

    If |v|=4, |w|=3 and the angle between and is pi/3, find |2v −w|

    2. Relevant equations
    ##cosθ=\frac {v dot w} {|v||w|} ##

    3. The attempt at a solution
    ## 6= v dot w ##

    This is as far as I got. How would I find the separate values of v and w for the equation?
  2. jcsd
  3. Jun 2, 2014 #2


    User Avatar
    Homework Helper

    Very good! How is the absolute value of a vector defined with dot product by itself?
    You do not need to know the individual vectors. You need the absolute value of 2v-w.

  4. Jun 2, 2014 #3
    u dot u = ##|u|^2## so |u|= ##\sqrt {u dot u} ## ?

    Possibly, that becomes |2v-w|=sqrt(something?) or would |?|(1/2)=|2v-w|

    I'm afraid I don't see how things are connecting to the third equation though.

    1/2 = (something analogous to u dot v)/|2v-w|?
    Last edited: Jun 2, 2014
  5. Jun 2, 2014 #4


    User Avatar

    Staff: Mentor

    Start from that definition. What then is ##\sqrt{(2v-w) \cdot (2v-w)}##?
  6. Jun 2, 2014 #5
    Got it, thanks.
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted