Question

Compare the potential energy of a 20 kg rock on the edge of 100 m cliff with that of a 20 kg rock perched on the edge of a 50 m cliff.

Answer 1

The equation that gives the potential energy of an object is given by:
$U=mgh$
where 
m is the mass of the object
g is the gravitational acceleration
h is the height above the ground

Using this equation, the potential energy of the first rock is:
$U_1 = mgh=(20 kg)(9.81 m/s^2)(100 m)=19620 J$
while the potential energy of the second rock is:
$U_2 = mgh=(20 kg)(9.81 m/s^2)(50 m)=9810 J$

So, the potential energy of the first rock is twice that of the second rock.