A stone of mass 2kg is dropped from the window of a high building, 45m above the ground.
a)What is the initial gravitational potential energy of the stone (relative to the ground)?
b)What is the kinetic energy of the stone at the moment just before it hits the ground?
c)At what speed does the stone hit the ground? Would the results have been different if the stone's mass had been 3kg?
...or just paranoid?