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Homework Help: Acceleration time problem

  1. Mar 7, 2010 #1
    1. The problem statement, all variables and given/known data
    An electron is accelerated uniformly from rest to a velocity of 2.0x10^7m/s(E) over the displacement .10m(E).

    How long does the electron take to reach its final velocity?

    2. Relevant equations

    This is what i want to know

    3. The attempt at a solution

    I know the answer, but can not figure out how to get it 1x10^-8
  2. jcsd
  3. Mar 7, 2010 #2


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    Maybe try using one of the kinematic equations like v=u+at or s=ut+1/2at2
  4. Mar 7, 2010 #3
    put u=0
  5. Mar 7, 2010 #4
    ok more precisely the problem is that i dont know acceleration or time. all the general equations i know, require either acceleration or time to figure out one or the other. So, not knowing either, how do I proceed?

    and, dont those three equations require me to know time??
  6. Mar 7, 2010 #5


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    you almost know the average speed :
    half the ending speed , because the starting speed was zero.
  7. Mar 7, 2010 #6
    When you don't know how to proceed, it's better if you list all the know data. Like this:

    [tex] v_{1}=0, v_{2}=..., \Delta d=..., a=?, \Delta t=?[/tex]

    Now, since you know v1,v2, Delta d, which formula can you use?
  8. Mar 7, 2010 #7
    of the 5 general equations for uniform acceleration that i know, all of the equations require us to know one of the two acceleration, or time.
  9. Mar 7, 2010 #8


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    well, if the equation doesn't have TIME in it , you'll never be able to solve for the time .

    But don't you have an equation withOUT acceleration?
  10. Mar 7, 2010 #9
    Interceptor has told you the answer.
  11. Mar 8, 2010 #10
    Ok, so if interceptors equation is,


    s is Speed = 1x10^7
    u is ? but = 0
    v is Velocity = 2x10^7
    t is time? = unknown

    Which means I cant proceed because I have an equation that requires one of my unknowns.

    Sorry for being so much trouble. But Im really havin a hard time with this lol.
  12. Mar 8, 2010 #11


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    s is displacement.
  13. Mar 8, 2010 #12
    It's alright. By s=(u+v)/2*t, Interceptor meant, s for displacement, u for initial speed and v for final speed. u=0 because the electron started from rest; other than that, you know s and v. Therefore, you can solve for t.
  14. Mar 8, 2010 #13
    ummm.. t= ((u+v)/2)/s ??? does that make sense? ahhhh I suck at this
  15. Mar 8, 2010 #14
    ahhhh Nice!


    Thanks so much guys! My heros!
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