A new role for the Cartesian diver: Showing free-fall weightlessness

The interior of a free-falling system, if it is small enough, can be thought of as a “gravity-free space” in accordance with Einstein’s equivalence principle, meaning all gravity-related forces, such as weight and the buoyant force, disappear. The last force lifts the Cartesian diver up from the bottom when the external pressure on a stationary water-filled bottle is released. Nevertheless, if the external pressure is released and, simultaneously, the bottle with the diver at the bottom is allowed to fall, the Cartesian diver will stay on the bottle’s bottom. This behavior of the diver is due to the fact that the interior of a free-falling bottle can be thought of as a gravity-free space.