Question 941995
If the sign were not negative, then the rotating
vector would form a  30-60-90 triangle in the 
1st quadrant. The cosine of 60 degrees is {{{ 1/2 }}} 
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As the vector rotates, the first time that the cosine
is {{{ -1/2 }}} is when
{{{ theta = 120 }}} or
{{{ theta = 2*pi/3 }}}