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Homework Help: Coulombs law, initial speed to collide with other nucleus

  1. Feb 14, 2013 #1
    1. The problem statement, all variables and given/known data

    A helium nucleus has radius rHe = 1.9 fm, mass m = 6.6 × 10-27 kg, and charge +2e. A gold nucleus has charge +87e and radius rAu = 7.0 fm. What initial speed must a helium nucleus have if it is to come into contact with a fixed gold nucleus in a head on collision?

    2. Relevant equations


    3. The attempt at a solution

    started by solving for potential energy by making U=k(q1q2/(r1+r2)) so i got U=4.51(10-12) J.
    Now setting this equal to (mv2)/2, i got
    and this came out as 3.697(107) m/s.
    The teacher's answer is 1.17(106) m/s.
    Could somebody help me find where i went wrong? this is due tomorrow i would be very grateful.
    1. The problem statement, all variables and given/known data

    2. Relevant equations

    3. The attempt at a solution
  2. jcsd
  3. Feb 14, 2013 #2


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    Your solution looks correct to me. The ratio of your answer to your teacher's answer is close to √1000. The mass of an alpha particle in grams is 6.6 x 10-24 g. I wonder if your teacher used a mass of 6.6 x 10-24 kg rather than 6.6 x 10-27 kg. That would account for the difference.
    Last edited: Feb 14, 2013
  4. Feb 14, 2013 #3
    Hm, that's interesting. I plugged in the 6.6(10-24) and got the answer the teacher provided. Thanks for noticing that. I guess I'll email him now...
  5. Feb 15, 2013 #4
    I've got the same answer as yours
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