Three balls of different masses are thrown at different instants up against gravity. While all the three balls are in air the centre of mass of the system of three balls has an acceleration:

lf m1, m2 and m3 be the masses of the balls, the acceleration corresponding to each ball is 'g'.Acceleration of centre of mass will bea = m1g +m2g+m3gm1+m2+m3 =g.this holds good as long as fragments are in air