Donald R. answered • 11/02/15
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High School Pysics, Math and SAT Math Prep
"... when it falls off the cliff" is Physicsspeak for that moment when it is no longer sitting on the cliff, but is in midair, but it hasn't started falling yet.
Realistically, this exists only for infinitely small moment of time. (like zero seconds.)
^{O < the boulder has not yet started to fall.}



But at that moment, the boulder has not yet started falling, so its velocity is 0. KE = 1/2mv^2, so with a velocity of zero the KE is 0J.