When an object is falling freely from the height h from the earth's surface its gravitational potential energy decreases gradually but its mechanical energy does not change.

Explanation

Because, the gravitational potential energy of the object at height from the earth.s surface is mgh. Now, when the object starts falling freely its height h from the earth.s surface decreases gradually. Hence, its gravitational potential energy goes on decreasing.
But, as the freely falling object performs constant accelerated motion `(a=g)` in downward direction its velocity goes on increasing uniformly. Hence as per the formula `E_(k)=(1)/(2)mv^(2)` its kinetic energy goes on increasing.
Now, as per the law of conservation of mechanical enregy `deltaE_(k)=-DeltaE_(p)`, the kinetic energy goes on increasing in proportion to the decrease in potential energy. Hence as per the formula `E=E_(p)+E_(k)`, the mechanical energy of the object does not change.