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!

Constant Acceleration Problem

  1. Sep 2, 2013 #1
    1. The problem statement, all variables and given/known data

    I am about to start AS Level Physics and have been given some questions/problems to solve as introductory homework. However, I am stuck on one and cannot seem to get the answer specified as the correct one.

    An electron of mass 9.11x10-31kg is accelerated by a positive charge such that the electron experiences a constant force of 8.5x10-29N. The electron, which was already moving, then travels 45mm in 35microseconds. By first determining the acceleration of the electron, calculate its initial velocity.

    2. Relevant equations

    F = ma
    s = ut + 1/2at2

    F = Force (N)
    m = mass (kg)
    a = acceleration (ms-2)
    s = distance travelled (m)
    u = initial velocity (ms-1)
    t = time (s)

    3. The attempt at a solution

    Converting Units

    45mm = 0.045m
    35microseconds = 0.000035s

    Working Out Acceleration

    F =ma
    8.5x10-29 = 9.11x10-31 x a
    8.5x10-29 / 9.11x10-31 = a
    a = 93 + 277/911

    Check: 9.11x10-31 x (93 + 277/911) = 8.5x10-29

    Working Out Initial Velocity

    Rearranging the Equation:

    s = ut + 1/2at2
    ut = s - 1/2at2
    u = (s-1/2at2) / t

    Putting in the Values

    u = (0.045 - 1/2(93+277/911) x 0.0000352) / 0.000035
    u = (0.045 - (1.142974755x10-7 x 1/2)) / 0.000035
    u = (0.045 - 5.714873765x10-8) / 0.000035
    u = 0.04499994285 / 0.000035
    u = 1285.712653

    According to the booklet, the answer should be 4290ms-1 and not the value I got.
    I have been working at this for hours and cannot seem to find where I am going wrong. Help me out?
  2. jcsd
  3. Sep 2, 2013 #2


    User Avatar

    Staff: Mentor

    Your work looks fine. Could be the booklet has a misprint: 4290 → 1290.

    With the given force the acceleration is on the order of 100m/s2. Over 35 microseconds that can only change the speed by a few thousandths of a meter per second (a*t). It won't significantly affect the initial velocity over that time period so a good approximation for the initial velocity is just Δs/Δt which is about 1290 m/s
  4. Sep 2, 2013 #3
    That is definitely a possibility gneill. I never tried rounding the numbers so never spotted how similar they actually could be. Thanks!
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted

Similar Discussions: Constant Acceleration Problem