Consider a container with lid of total mass, $M$. A mass, $m$, is allowed to fall into the container when the container is open. The whole set up is kept on a sensitive weighing machine and everything is in vacuum.
When the mass, $m$, is inside the container the container is closed with the lid not letting any other matter to enter or escape the container.
Neglecting any effect of the movement of the lid, will the weighing machine show the same or different readings during the two stages, when the container is open and when it is closed before the mass $m$ hits the bottom of the container? Why? If there was air instead of vacuum, in the second stage the weighing machine would also measure the weight of the air trapped. But apart from this, would the situation be any different?