I have been doing some thought experiments, and would like some help in thinking about two situations:
a) There is a (metallic) container on a weighing machine. This container has an upper chamber and a lower chamber, separated by a weightless partition that can be removed from outside. The lower chamber has some gas under pressure. If I remove the partition, the gas will expand into the upper chamber... will this expansion create a thrust against the upper wall of the container that will reduce the weight measured by the weighing machine ? Perhaps the thrust is enough to launch the container upwards ? What if instead of removing the partition, I just create a hole in it (from outside), so that the gas goes through the hole like a stream ?
b) There is a container with ideal gas on a weighing machine, but the upper wall of the container is a piston that can be pushed down. If I push the piston downwards impulsively, the gas will compress, but will the weight recorded by the weighing machine transiently increase ?