A 45.2kg boulder falls off a mountain that is 255 m tall. What is the kinetic energy of the boulder when it falls off the cliff?

Question

I know the answer might be 0J but I need help explaining how to get the answer

Answer 1

"... when it falls off the cliff" is Physics-speak for that moment when it is no longer sitting on the cliff, but is in mid-air, 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.