If you have a light bulb that weighs 10 g when empty, how does adding an ideal gas inside of it change the mass? If I'm not mistaken, the force the gas exerts on the light bulb (well, a perfectly symmetrical object is a better example) is the pressure, and it would be pointing in all directions. Overall, all the upward and downward vectors (as well as others) of the force of the individual gas molecules would cancel, right? So then, how exactly would the force of gravity of this system be different from 10 g if the Fup-gas = Fdown-gas, just in opposite directions? Wouldn't this cancel the effects of the light bulb being pushed down since the net force from the gas (the weight from the gas) is 0.