Question

when the object is certain height from the surface of the earth ,the acceleration due to gravity decreases ?why???

Answer 1

because of the heights are increased

Answer 2

As the gravity formula -

(~ = proportion sign)

g ~ m/r2

where g is the gravity , m is the mass, and r is the radius, so if you go high in altitude you will increase the radius and gravity is inversely proportional to the square of the radius , so more the radius lower the intensity of gravity.