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

Equations for relative velocity

  1. Nov 28, 2012 #1
    hello everyone,

    Can someone kindly show me the equation for relative velocity calculations?

    when i search for them via wikipedia, there is only one formula for collinear objects and they move to the same direction...

    so what if they move opposite to each?

    and what if their motions follow a curvature ...:confused:

    plz help :P
  2. jcsd
  3. Nov 28, 2012 #2


    User Avatar
    Staff Emeritus
    Science Advisor

  4. Nov 28, 2012 #3


    User Avatar
    Science Advisor

    The equation you mention for "relative velocity" is: if object A is moving away from me at velocity u and object B is moving along the same line and in the same direction at speed v (both relative to me) then the speed of object A relative to B is given by
    [tex]\frac{u- v}{1+ \frac{uv}{c^2}}[/tex]

    If they are moving in opposite directions, replace "-v" with "+v".

    If they are not moving along the same line, draw a line between them and calculate their components of velocity parallel and perpendicular to that line. Apply the formula above to the parallel components only.
  5. Nov 28, 2012 #4


    User Avatar

    Staff: Mentor

    And replace "+v" with "-v". More concisely, reverse the sign of v everywhere in the equation.
  6. Nov 28, 2012 #5
    thank you for your reply!

    unfortunately, when I search on website, I was told the equation as:

    [tex]\frac{u- v}{1- \frac{uv}{c^2}}[/tex]

    and it says if they go opposite directions, I need to replace the sign...if you think im paranoid :) , plz check here http://en.wikipedia.org/wiki/Relative_velocity

    to be honestly, im not a guy with physics background, i just like it.... so this is confusing for me... could you plz help?

    once again, thank you!
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook