Abstract: A simple experiment gives an interesting phenomenon of which an object system in static state can show overweight and weightlessness. Such a complicated object system is designed, including container, water, and balloon, of which the weight of the floating body is unequal to the interplay force between it and water. Because of the change in the thermodynamic state of the balloon results from change in the floating force and hydraulic pressure, then reversely causing change in buoyancy force, finally, the platform balance shows overweight and weightlessness compared with the total weight of the object system. The floating body spontaneously changes its movement in upright direction from the non–balance to balance states with the macro–entropy increasing, under of which we propose the concept of macro–entropy change because of work done by gravity, developing the idea of entropy change for the mechanical movement. What the increases in the macro–entropy change means the spontaneity of change in state, as well as the difference between the self–weight of the moving object and the interplay between it and water, by which we can judge the overweight and weightlessness. Furthermore, it can also make a demonstration on how to analyze the change in mechanical state and force.