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Homework Help: Collisions -- conceptual questions

  1. Apr 6, 2017 #1
    1. The problem statement, all variables and given/known data
    Let two particles of equal mass m collide. Particle 1 has initial velocity v, directed to the right, and particle 2 is initially stationary.

    A: If the collision is elastic, what are the final velocities v_1 and v_2 of particles 1 and 2?

    B: Now assume that the mass of particle 1 is 2m, while the mass of particle 2 remains m. If the collision is elastic, what are the final velocities v_1 and v_2 of particles 1 and 2?

    2. Relevant equations
    mv_1 + mv_2 = mv
    ii: 0.5m(v_1)^2 + 0.5m(v_2)^2 = 0.5mv^2
    iii: 2mv_1 + mv_2 = 2mv
    iv: 0.5(2m)(v_1)^2 + 0.5m(v_2)^2 = 0.5(2m)v^2

    3. The attempt at a solution
    For A, after factoring out m and rearranging equation i to solve for v_2 I replaced v_2 in equation i with v_2 = (v - v_1) to yield:
    v_1 + v_2 = v_1 + (v - v_1) = v_1 - v_1 + v = v, therefore
    v = v.

    I guessed (correctly) that v_1 = 0 and v_2 = v, but I'm not sure how I can find those answers using this equation.

    It's the same thing with part B.

    I tried rearranging the kinetic energy equations ii and iv for both parts A and B but still came out with such useless results as v^2 = v^2.

    Why does particle 1 in part A transfer all of its momentum to particle 2?
    Last edited: Apr 6, 2017
  2. jcsd
  3. Apr 6, 2017 #2


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    Homework Helper

    Substitute v_2 = (v - v_1) into the energy equation.

    Again, use both equations, momentum and energy. Express v_2 with v_1 from the momentum equation and substitute into the energy equation.
    It is the solution of the system of equation i and ii.
  4. Apr 9, 2017 #3
    That makes sense, thanks!
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