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Finding the speed of alpha

  1. Nov 16, 2008 #1
    1. The problem statement, all variables and given/known data
    A beam of α particles with energy/particle = 2.86 MeV is directed towards lead foil.
    What is the speed of the α particles?

    2. Relevant equations
    E = 1/2 mv^2
    v = SQRT(2E/m)
    v = SQRT((2)(2.86E+6)(1.602E-19) / (1.673E-27) ) = 23403526 m/s = 0.078c
    but the ans is 0.039c which is half of the answer above. Why??

    3. The attempt at a solution
  2. jcsd
  3. Nov 16, 2008 #2


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    Staff Emeritus
    Science Advisor
    Gold Member

    Did you do your calculations right? Did you use the right constants?

    (2 * 2.86 MeV / (4 * (proton mass)))^(1/2) / c​
    into google gives me the correct answer. (I've assumed the difference in proton and neutron masses are negligible)

    Incidentally, I'm assuming you are neglecting relativistic effects?
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