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Homework Help: Collision in two dimensions

  1. Apr 1, 2012 #1
    1. The problem statement, all variables and given/known data

    A neutron (mass 1.7x10-27 kg) traveling at 2.7 km/s hits a stationary lithium nucleus (mass 1.2x10-26 kg). Afterwards, the velocity of the lithium nucleus is 0.40 km/s at 54° to the original direction of the motion of the neutron. If the speed of the neutron after the collision is 2.5 km/s, in what direction is it traveling?

    [Answer: 66° from initial direction of neutron's velocity]

    2. Relevant equations


    m1= 1.7x10-27 kg
    v1x= 2.7 km/s

    v'1x= (2.5)cosθ
    v'1y= -(2.5)sinθ

    m2= 1.2x10-26 kg
    v2x= 0 km/s

    v'2x= (0.4)cos54°
    v'2y= (0.4)sin54°


    m1v1x= m1v'1x+m2v'2x

    0= m1v'1y+ m2v'2y

    3. The attempt at a solution

    I plugged in my values into the two equations, and for the first equation using the x-values I got θ= 65.42° and the second using the y-values I got 65.51°. I was wondering if anyone could point me in the right direction?

    I've attached a diagram of my thought process.
    1. The problem statement, all variables and given/known data

    2. Relevant equations

    3. The attempt at a solution

    Attached Files:

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  2. jcsd
  3. Apr 1, 2012 #2


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    Staff: Mentor

    The values given are probably not very precise, and very probably rounded; they are specified with only two significant digits. Accordingly, you can expect a little "slop" in the results. Believe only two digits, with the second being a bit "fuzzy" :smile:
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