1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: 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.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook