Abstract: When a light and a heavy balloon are arranged in a transparent thin shell box. the light

one floated up and the heavy one sunk. They are tied by a filament to fix on the upper and lower

shell walls，so that the central heights of the two balloons are roughly equal. The thin shell

box is fixed on a flat table car. The flat table car is pushed to accelerate along the direction of the connecting line between their centers and they can

make opposite or separated acceleration motion under the joint action of equivalent gravity

and inertial force. This experiment can help students to understand the concept of gas heat

balance，equivalent principle，inertial force and handling prob- lems in non-inertial system.

Key words: thermal equilibrium of gas, equivalent principle, inertial force, non inertial system, Newton's law