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Homework Help: Electron - Vector Problem

  1. Sep 12, 2004 #1
    One part of this problem has me confused....id appreciate any guidance. I have part (a) and (b) but (c) and (d) are TOTALLY losing me. i don't even know where to begin. THANKS :smile:

    "An electron's position is given by r=3.00t i - 5.00t^2 j + 3.00 k, with t in seconds and r in meters"

    (a) In unit-vector notation, what is the electron's velocity v(t)?
    My answer: 3.00 i - 10.0t j+ 0.00 k

    (b) What is v in unit-vector notation at t=6.00s?
    My answer: 3.00 i - 60.0 j+ 0.00 k

    (c) What is the magnitude of v at t = 6.00 s? :eek:

    (d) What angle does v make with the positive direction of the x axis at t = 6.00 s? :eek:

    Thank you.
  2. jcsd
  3. Sep 12, 2004 #2


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    Science Advisor
    Homework Helper

    To find the magnitude of a vector just square each component, add them up and find the square root.

    You can find the angle between two vectors using the "dot product:"

    [tex]\vec A \cdot \vec B = A B \cos \phi[/tex]

    where A and B are the magnitudes of the vectors and [itex]\phi[/itex] is the angle between them.
  4. Sep 12, 2004 #3
    thanks.... solved it :-)
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