Question

Why we assume that the magnitude of gravitational accelaration is constant (9,8 m/s2) although it is not in reality?

Answer 1

It is.

Answer 2

if we cnsider air resistyance it makes it miserable to calculate basic computations like height

Answer 3

On the surface of the earth its variation is minimal and is assumed to be constant for ease of calculation. As suggested by the eqn:
g=GM/R^2
It varies inversely to the square of the distance from centre of the earth.
But when you are dealing with airplanes and rockets this variation is taken into account.

Answer 4

ya you are correct it always varies with height, altitude , longitude ,but for our simplicity we we don't take in comparison as masses we consider are very small as compared to earth.