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Homework Help: Vector motion in a circle

  1. Feb 24, 2012 #1
    1. The problem statement, all variables and given/known data



    3. The attempt at a solution

    I understand how to get the derivatives for velocity and acceleration but when the book plugs in the numbers I disagree.

    velocity at pi
    2 sin pi/2 = 0
    2 cos pi/2 = -1j
    therefore, 0 + - 1j = -1j (the books says -2i)

    velocity at (3pi)/2
    2 sin 3pi/2 = 0
    2 cos 3pi/2 = 1i
    therefore, 0 + 1i = 1i (the book says -√2i - √2j)

    acc at pi
    -cos pi/2 = 1/2i
    -sin pi/2 = 0
    therefore, 1/2i + 0 = 1/2i (the book says -1j)

    acc at (3pi/2)
    -cos 3pi/2 = 0
    -sin 3pi/2 = 1/2j
    therefore, 1/2j (the book says -√2/2i - √2/2j)
    Last edited: Feb 24, 2012
  2. jcsd
  3. Feb 24, 2012 #2


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    Science Advisor
    Homework Helper

    hi bobsmith76! :smile:

    (have a pi: π :wink:)

    hmm …
    sinπ/2 = 1
    cosπ/2 = 0
    no, you need 3π/4 :redface:

    get some sleep! :zzz:
  4. Feb 24, 2012 #3
    I highly recommend you review your trig and algebra.

    sin(pi/2)=1 and cos(pi/2)=0.

    That's not what you need to calculate. You need to calculate -2sin(3pi/4) and 2cos(3pi/4).

    cos(pi/2)=0 and sin(pi/2) = 1.

    That's not what you need to calculate. You need -cos(3pi/4) and -sin(3pi/4)
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