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: How do I use this confusing elastic collision equation?

  1. Dec 22, 2012 #1
    Hi guys, this is the equation of relative velocities in elastic collisions:
    Va1x-Vb1x = -(Va2x-Vb2x)
    I know that this means that the relative velocity of A to B before an elastic collision is the same in magnitude but opposite in direction after the collision. However, this applies only in the x direction (for this particular equation). My questions are:

    1. Can it be applied to y and z directions?

    2. If so, then if initially A and B are of different velocity in 2D (x and y directions only) and I compare their relative velocities of x and y before and after, does the equation applies?

    3. If initially A is only moving in the x direction and B is at rest, and after the elastic collision A and B are scattered in an x-y plane, does the equation for relative velocity of y still apply? Note that initially A does not have any y velocity so if I apply the equation, they should have equal y velocities magnitude. Yet if they don't have the same mass, then the conservation of momentum in the y direction is broken since there's a net y momentum from 0 y momentum initially (note that the question doesn't have external forces at all) does this mean that it is not possible to use this equation in y and z direction? So that a system without initial net y velocity or momentum cannot use the equation given above even if its an elastic collision??

    Thanks a lot guys!!
  2. jcsd
  3. Dec 22, 2012 #2


    User Avatar
    2017 Award

    Staff: Mentor

    That equation works for one-dimensinal collisions only.
    In two (or more) dimensions, you can get similar equations using momentum and energy conservation.
  4. Dec 22, 2012 #3
    Oh ya i just realised that. That equation don't work for anything more than 1D. Thanks!
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook