m = mass of the ball at the edge of the cliff = 2 kg
h = height of the cliff from where the ball falls off = 40 m
g = acceleration due to gravity = 9.8 m/s²
gravitational potential energy of the ball at the top of the cliff is given as
PE = mgh
inserting the above values in the formula
PE = (2) (9.8) (40)
PE = 784 J
KE = kinetic energy of the ball as hits the ground.
initially at the top of cliff , the ball has no kinetic energy and only potential energy. as the ball falls down the cliff , the speed of ball increase which increases the kinetic energy and potential energy decrease by same amount as the height above the ground decrease.
using conservation of energy
kinetic energy of ball as it hits the ground = Potential energy of the ball at the top of cliff
KE = PE
KE = 784 J