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: Velocity from an elastic collision

  1. May 13, 2017 #1
    1. The problem statement, all variables and given/known data
    A body X moving with a velocity v makes an elastic collision with a stationary body Y of equal mass on a smooth horizontal surface. Which statement gives the velocities of the two bodies after the collision? (multiple choice question)

    3. The attempt at a solution
    The answer according to the mark scheme is
    velocity of X = 0
    velocity of Y = v

    So I can't understand how this is worked out. But this as far as I got:

    MUx+MUy = MVx + MVy
    Ux +Uy = Vx +Vy
    Ux = Vx + Vy

    How do you know that Vx is 0 and Vy is V (or Ux)?
  2. jcsd
  3. May 13, 2017 #2
    You've used the conservation of momentum which is true for all collisions. Since the question specifies that it's an elastic collision, you know that kinetic energy will be conserved. Try including that in your equations.
  4. May 13, 2017 #3
    Thank you, I understand now!
  5. May 13, 2017 #4


    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    When you have conservation of both energy and momentum the two equations combined produce this neat result: the relative velocity reverses. I.e. v1f-v2f=v2i-v1i. See "Newton's Experimental Law" for a generalised version.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted