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Homework Help: Newton's Laws of Motion problem help

  1. Aug 12, 2010 #1
    Hi everyone! Im really bad in physics so I need help in my assignments. My problem is what formula to use. Here's the problems.

    1. The problem statement, all variables and given/known data
    An electron (mass = 9.11 x 10-31 kg) leaves one end of a tv picture tube with zero (0) initial speed and travels in a straight line to the accelerating grid, which is 1.80 cm away. It reaches the grid with a speed of 3.00 x 1006 m/s; If the accelerating force is constant, compute:
    a.) acceleration
    b.) time to reach the grid
    c.) net force in Newtons (gravitational force on the electron may be neglected)

    m = 9.11 x 10-31
    vo = 0 m/s
    vF = 3.00 x 1006
    d = 1.80 cm = 0.018m

    In A:
    I've thought of two formulas. Both give different results. These are:

    a = v/t

    Since there is no TIME, I will have to solve for time using:
    t = m(v - vo) / F

    The other formula I have thought of is:
    a = v2 / 2x (from v2 = vo2 + 2ax)

    In B:
    It's either:
    t = m(v - vo) / F
    t = d/v

    both giving different results again

    In C:
    It's obviously F = ma...

    Please help me, I really don't know what formula to use.
  2. jcsd
  3. Aug 12, 2010 #2


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    To calculate acceleration a, use v^2 = vo^2 + 2ax

    To find t, use v = vo + at.
  4. Aug 12, 2010 #3
    Thank you.
    About solving for t, my teacher didn't give that kind of formula, is that a formula from Force?
  5. Aug 12, 2010 #4


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    Gold Member

    You already give a=v/t which is the same as v=v0+at for v0=0. This formula comes from the definition of acceleration a=dv/dt, if we know that a is constant .
  6. Aug 12, 2010 #5
    oh ok thanks. ^^
  7. Aug 12, 2010 #6


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    Homework Helper

    The formula for t is derived from the definition of the acceleration.
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