A body is falling freely under the action of gravity alone in vacuum. Which one of the following remains constant during the fall?

When a body falls freely under the action of gravity alone in a vacuum, it is subject only to a conservative force. According to the Law of Conservation of Mechanical Energy, the total mechanical energy (the sum of kinetic and potential energy) remains constant throughout the fall. As the body descends, its gravitational potential energy decreases because its height above the ground reduces. Simultaneously, the body accelerates due to gravity, causing its velocity and kinetic energy to increase. While kinetic energy and potential energy change individually, their sum remains invariant in the absence of non-conservative forces like air resistance. Total linear momentum does not remain constant because the external gravitational force causes the body's velocity to change over time. Therefore, only the total mechanical energy is conserved during the motion.

Sources

1. [1] https://www.grc.nasa.gov/www/k-12/VirtualAero/BottleRocket/airplane/ffall.html