Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Vector distance formula

  1. Jul 25, 2007 #1
    let P be a point NOT on line L that passes through points Q and R.

    [tex] \vec{A} = QR[/tex]

    [tex]\vec{B} = QP [/tex]

    prove that distance from point P to anywhere on line L is
    [tex] d = |\vec{A} x \vec{B}| divided by |\vec{A}| [/tex]

    so, i've tried doing the cross product after assigning variables for the A and B components. I ended up with a very tedious long multiplication of several variables, and I was wondering if there is an easier way to prove this formula.
    Last edited: Jul 25, 2007
  2. jcsd
  3. Jul 25, 2007 #2
    Draw a picture of what is going on and note that |AxB| is the area of the parallelogram generated by A and B. It's also equal to |A||B|Sin(t) where t is the angle between A and B.
  4. Jul 25, 2007 #3


    User Avatar
    Science Advisor

    Of course the shortest distance from P to a line is along the line through P perpendicular to the line. You might start by finding the equation of a line through P perpendicular to [itex]\vec{QR}[/itex].
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook