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Please help a simple problem giving big problems

  1. Feb 13, 2007 #1
    A proton and an alpha particle (q = +2.00e, m = 4.00u ) are fired directly toward each other from far away, each with an initial speed of 0.141c. What is their distance of closest approach, as measured between their centers? (Hint: There are two conserved quantities. Make use of both.)

    I would figure that you could use conservation of energy in the sense that the energy of the system initially is the kinetic energies of the two particles combined (Enet1 = Kp + Ka). At the point of closest approach, their speeds should be zero, and hence Enet2 = Uelec = Kq1q2/r. From here it should be straightforward:

    Enet1 = Enet2
    Kp + Ka = Kq1q2/r

    Then solve for "r".

    However, this is incorrect. Perhaps my assumption that the alpha particle (4 times the mass, 2 times the charge) stops completely is wrong. At this point, I really have no idea.
  2. jcsd
  3. Feb 13, 2007 #2
    It might work better if you set it up in a frame where the center of mass is stationary.

    Then use the conservation of energy and momentum.
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