A simple experiment shows an interesting phenomenon of an object system in apparently static state exhibiting overweight and weightlessness. Such a complicated object system includes container,water,and balloon,among which the weight of the floating body is unequal to the interplay force with water once the equilibrium state is broken. Because of change in the thermodynamic state of the balloon,the buoyancy force and hydraulic pressure also change with it,directly resulting in that the platform balance shows overweight and weightlessness. The floating body spontaneously changes its movement in upright direction from the non-equilibrium to equilibrium states accompanied by an increase in entropy. Based on it,the concept of macro-entropy change due to mechanical work done by gravity is proposed. Furthermore,the increases in the macro-entropy change mean the spontaneity of change in state,and the existence of difference between the self-weight of moving object and the interplay with water,by which we can judge the overweight and weightlessness. Meanwhile,we also give a demonstration on how to analyze the changes in mechanical state and forces for complicated object system.