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Homework Help: Confused on which equation to use (simple question)

  1. Mar 8, 2009 #1
    1. The problem statement, all variables and given/known data
    A uniform electric field exists in the region between two oppositely charged parallel plates (1.64*10-2)m apart. A proton is released from rest at the surface of the positively charged plate and strikes the surface of the opposite plate in a time interval (1.60*10−6)s.

    Find the speed of the proton at the moment it strikes the negatively charged plate.

    2. Relevant equations
    [tex]x = x_0 + v_0 t + (1/2) a t^2[/tex]
    [tex]v = v_0 + a t[/tex]
    [tex]v^2 = v_0^2 + 2 a (x - x_0)[/tex]

    3. The attempt at a solution
    Okay so I found a by using this equation ([tex]x = x_0 + v_0 t + (1/2) a t^2[/tex]) and it came out to be 1.2812*10^10.
    To find velocity when it hits, I realized that I get a different answer when I use this ([tex]v = v_0 + a t[/tex]) and that ([tex]v^2 = v_0^2 + 2 a (x - x_0)[/tex]). The correct answer came out when I used the latter equation.

    So my question is, why is it that I can't use the first equation? Shouldn't they come out to be the same answer? Did I do something wrong?

    Thanks guys.
    Last edited: Mar 8, 2009
  2. jcsd
  3. Mar 8, 2009 #2


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    Homework Helper

    Are you sure you just aren't making a mistake in the calculation? I get the same answer both ways.
  4. Mar 8, 2009 #3
    oh i feel stupid. thank you very much haha i calculated it couple times before i posted it but I guess I had a mistake in it.

    Man, it took me awhile to write this too.

    anyways, thanks. :)
  5. Mar 8, 2009 #4


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    Homework Helper

    lol you're welcome.
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