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Homework Help: Newton's Laws and an electron of mass

  1. Feb 1, 2008 #1
    1. An electron of mass 9.11 x 10^-31 kg has an initial speed of 4.00 x 10^5 m/s. It travels in a straight line, and its speed increases to 6.00 x 10^5 m/s in a distance of 3.00 cm.
    (a) Assuming its acceleration is constant, determine the force exerted on the electron.
    N (in the direction of motion)
    (b) What is the ratio of this force to the weight of the electron, which we neglected?

    2. F=ma
  2. jcsd
  3. Feb 1, 2008 #2
    Use one of the kinematics equations to determine the accelartion

    You have initial velocity, final velocity and distance.
  4. Feb 1, 2008 #3
    What you know is this, in 0.03 meters of travel you achieved an additional velocity of 2 x10^5 m/s. You also know that s=0.5*a*t^2 thus 0.03 = 0.5*s*t^2 (first equation).

    Next you know that v=at and that the increase of velocity was 2x10^5, thus the time could be found by using 2x10^5 = a*t (second equation)

    You have two equations with two unkowns, solve for "a" and "t" then use F=ma to get the Force in Newtons. Acceleration will be about 66.67m/s^2 and F about 6.07X10^-24.

    Hope this helps.
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