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Homework Help: Finding the kinetic energy of an electron

  1. Apr 18, 2012 #1
    1. The problem statement, all variables and given/known data

    An electron's position is given by r(t)=r(0)+c1*t, θ(t)=θ(0)+c2*t, where r(0), c1, θ(0), and c2 are known constants. What is the kinetic energy of the electron? (mass=m)

    2. Relevant equations

    KE=(1/2)Iω2=(1/2)mr2ω2

    3. The attempt at a solution

    plugging in m for mass, r(0)+c1*t for radius, and c2 for ω
    (1/2)(m)(r(0)+c1*t)2(c2)2

    This isn't the correct answer, so I'm fairly certain I've messed up on my "r" value somehow.

    It's supposed to equal (1/2)(m)(c12+r2c22)

    Where am I going wrong?
     
  2. jcsd
  3. Apr 18, 2012 #2

    tiny-tim

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    Homework Helper

    hi texan14! :smile:
    what on earth are you doing? :confused:

    that's for a rigid body with angular velocity ω

    this is a spiral … you have no idea what ω(t) is, it certainly isn't θ'

    start again, and just use v = (r',rθ') :smile:

    (you do know how to prove that, don't you? … if you don't, then look it up)
     
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