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Homework Help: Electron Beam

  1. Sep 6, 2009 #1
    1. The problem statement, all variables and given/known data

    In a TV set, an electron beam moves with horizontal velocity of 2.5 x 107 across the cathode ray tube and strikes the screen, 40 cm away.

    The acceleration of gravity is 9.8 m/s2.

    How far does the electron beam fall while traversing this distance? Answer in units of m.

    2. Relevant equations

    x = voxt

    vy = voy - gt

    3. The attempt at a solution

    I solved for t:

    x = voxt

    .04 = 2.5 x 107
    t = 1.6 x 10-9

    Then I plugged t into vy = voy - gt.

    vy = 0 - (9.8)(1.6x10-9)
    vy = -1.568x10-8

    Now I am not sure if I have the correct values plugged in because I do not think I am supposed to come up with a negative answer. I did draw a picture but am not sure if my math corresponds to the picture.

    Any suggestions are appreciated,
  2. jcsd
  3. Sep 6, 2009 #2


    User Avatar
    Homework Helper

    vy = voy - gt
    Check this formula.
    If vy and g are in the same direction, they must have the same sigh.
  4. Sep 6, 2009 #3
    Thanks for the quick reply. I will think about it while I sleep and report back tomorrow morning.
  5. Sep 7, 2009 #4
    I figured out my mistake. I divided the 40 cm distance by 1000 instead of 100 which gave me the incorrect time. I solved for time again and plugged that into y = 1/2gt^2 which gave me the correct answer.
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