Question

Why does the acceleration changes when the velocity change ?

Answer 1

Acceleration is defined as the rate of change of velocity.
$\mathrm{whenever}\mathrm{there}\mathrm{is}\mathrm{a}\mathrm{change}\mathrm{in}\mathrm{velocity}\mathrm{with}\mathrm{time},\mathrm{there}\mathrm{must}\mathrm{be}\mathrm{an}\mathrm{acceleration}.$