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Homework Help: Collisions and the Speed of Two Pucks

  1. Mar 5, 2009 #1
    1. The problem statement, all variables and given/known data
    A hockey puck moving along the +x axis at 0.5 m/s collides into another puck that is at rest. The pucks have equal mass. The first puck is deflected 37degrees below the +x axis and moves off at 0.36 m/s. Find the speed and direction of the second puck after the collision.

    2. Relevant equations
    1/2*m_a*v_ai^2 + 1/2*m_b*v_bi^2 = 1/2*m_a*v_af^2 + 1/2*m_b*v_bf^2
    m_a*v_ai + m_b*v_bi = m_a*v_af + m_b*v_bf

    3. The attempt at a solution
    For part1, I am using the condensed formula of:
    v_ai^2 = v_af^2 + v_bf^2
    since the masses are equal and the second puck starts at rest, which gives me
    v_bf = .3496 (says it is incorrect)

    For part2, I know to separate the x-component and y-component to get (condensed again):
    0.5 = (.36)(cos37) + (v_bf)(cos[unknown])
    0 = (.36)(sin37) + (v_bf)(sin[unknown])

    I could solve for the unknown angle if I knew v_bf.

    Can anyone help me find what I am doing wrong?
  2. jcsd
  3. Mar 5, 2009 #2


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    Homework Helper

    Hi bowbe,

    Did you give the whole problem above? This equation is for elastic collisions (kinetic energy conserved). Is this an elastic collision?
    Here you have two equations with two unknowns, and so you can solve for both unknowns from just these two. (However, I would write this with one of these terms being negative.)
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