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Homework Help: Conservation of momentum and energy problem (Please check if my setup is right)

  1. Feb 8, 2012 #1
    1. The problem statement, all variables and given/known data
    Untitled_picture.jpg


    2. Relevant equations
    m1v1i + m2v2i = m1v1f + m2v2f (Conservation of momentum)
    (1/2)m2*(v2i)^2 + (1/2)m1*(v1i)^2 = (1/2)m2*(v2f)^2 + (1/2)m1*(v1f)^2 (Conservation of energy)


    3. The attempt at a solution
    I separated the momentum into x and y components and got 2 equations
    I used the conservation of energy (can be used since its elastic) and got another equation
    -I have 3 unknowns and three equations...which gives me hope that this is solvable lol
    1. The problem statement, all variables and given/known data
    Here are my equations:
    Momentum for x:
    0 = m2*v2f*sin(theta) + m1*v1f*sin(75)
    unknowns here: theta, v2f, v1f

    Momentum for y:
    m2*v2i = m2*v2f*cos(theta) + m1*v1f*cos(75)
    unknowns here: theta, v2f, v1f

    Conservation of energy equation:
    (1/2)m2*(v2i)^2 = (1/2)m2*(v2f)^2 + (1/2)m1*(v1f)^2
    unknowns here: v2f, v1f


    Can you guys see if these equations are right before I start using the tedious algebra involved in this.
     
    Last edited: Feb 8, 2012
  2. jcsd
  3. Feb 8, 2012 #2

    vela

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    The x-momentum equation should have a minus sign before the first term, but other than that, your set-up looks fine.
     
  4. Feb 8, 2012 #3
    oh so..
    0 = -m2*v2f*sin(theta) + m1*v1f*sin(75)?
    Minus sign just because it goes to the left right?
    And thank you!
     
  5. Feb 9, 2012 #4

    vela

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    Right.
     
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