1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

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


    User Avatar
    Homework Helper

    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


    User Avatar
    Homework Helper

    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


    User Avatar
    Homework Helper

    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


    User Avatar
    Homework Helper

    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!
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook