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Homework Help: Conservation of Energy question :cry:

  1. May 10, 2007 #1
    1. The problem statement, all variables and given/known data
    A proton 2nm away from an oxygen nucleus is launched towards the nucleus with a velocity of 3.2E4 m/s. What is the closest approach?

    2. Relevant equations
    # of protons in oxy = 8
    Ek =.5 mv^2
    U = (kqQ)/(r)

    3. The attempt at a solution
    I know the answer to this question is 0.57 nm, and I also got this answer when I did this question very first time (that was like last week) however, now whenever I try this question I get 1.04 nm.

    I am using this equation
    change in Ek = work

    -0.5mv^2 = - (9E9 x 1.6E-19^2 x 8) (1/Rf - 1/Ri)

    0.5(1.67E-27)(3.2E4)^2/(9E9 x 1.6E-19^2 x 8) + 1/2nm = 1/Rf
    Rf = 1.04E-9

    I know I am making some mistake, but just cannot figure it out:cry:
  2. jcsd
  3. May 10, 2007 #2
    [tex]E_{k} = qV = q\frac{KQ}{r}[/tex]
    find r
    q = proton (p) (move)
    Q = charge against proton (8p)

    in work or energy calculation not use negative
    Last edited: May 10, 2007
  4. May 11, 2007 #3
    after spending like more than 2 hours, I figured out that when I was copying this question from the black board, I copied the value of v wrong, and I didn't make correction in my notes though wrote the correct answer.
    So, that screwed up everthing.

    Anyways, thanks a lot.
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