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Homework Help: 2-d momentum collision

  1. Dec 13, 2009 #1
    1. The problem statement, all variables and given/known data

    Two hockey pucks of equal mass undergo a collision
    on a hockey rink. One puck is initially at rest, while
    the other is moving with a speed of 5.4 m/s. After the
    collision, the velocities of the pucks make angles of
    33° and 46° relative to the original velocity of the
    moving puck. Determine the speed of each puck after the
    collision.

    V1i=5.4m/s
    m1=m2
    V2i= 0
    V1'=?
    V2'=?

    2. Relevant equations


    P=P'
    M1V1+M2V2=M1V1'+M2V2'
    Eki=Ekf
    1/2mv1i^2+1/2mv2i^2=1/2mv1'^2+1/2mv2'^2


    3. The attempt at a solution


    k so i understand we have two unknowns and thus we should have two unknown equations.

    so ..

    M1V1+M2V2=M1V1'+M2V2'

    masses equal so they can be cancelled
    and we know V2=0 so that whole part is removed

    v1= v1'+v2'

    5.4= v1'+v2'


    5.4-v1'=v2'

    ^^ first unknown equation

    now when i place it into 1/2mv1i^2+1/2mv2i^2=1/2mv1'^2+1/2mv2'^2
    it does not give me the right answer or better yet i do not know how to continue on from this i always get

    5.4-v2^2=5.4-v2^2+v2'^2


    can someone please explain ...thank you in advance.
     
  2. jcsd
  3. Dec 13, 2009 #2

    kuruman

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    This is a two-dimensional collision so you need to conserve momentum independently along the original direction of the puck and along a direction perpendicular to it. The problem does not mention that kinetic energy is conserved, so you may not assume that it is.
     
  4. Dec 14, 2009 #3
    oh okay thank you i will try to figure it out
     
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