# Reduced-gravity aircraft

The words weightlessness, zero gravity and microgravity are often misused in everyday language.

• "Weightlessness" means "lack of weight" which should not be confused with "absence of gravitation" because there is theoretically no region in the universe where there is no gravitational field, however small. On the other hand, for an individual, "feeling" the force of gravity (weight) passes by the compensation effect that the reaction force of the support exerts on us. From this point of view, "free fall" is the only true state of weightlessness since an object no longer undergoes any reaction from the support (if the support is also in free fall with the object).
• "Microgravity" characterizes a region where the force of gravity is negligible compared to that felt on Earth. A region of space far enough from any mass verifies this condition. This is the case, for example, of a body located more than 200,000 km from the Earth (beyond the lunar orbit). The term "microgravity" is therefore rarely appropriate in physics, as is the name "Zero-G" of the aircraft. Neither the passengers of the Zero-G aircraft nor even the astronauts of the ISS are in microgravity. They undergo a gravitational force even if they no longer feel their own weight. For proof, the acceleration value of g in the ISS is g = 9 m.s-2 and is finally very close to the value of g at the Earth’s surface, g = 9.8 m.s-2.

If one applies the equivalence principle as stated by Einstein and illustrated by his elevator experiment, there is total equivalence between a body in free fall and the same body in weightlessness.

The common point between each of the three examples in this animation is that each "guinea pig" finds itself in weightlessness when it is in free fall. He no longer suffers his own weight. The same would apply to passengers at the orbital station or even the Moon who seem to float above our heads while they are just in "permanent" free fall. Strangely enough, the same is true for all of us when we jump in the air.

See also eduMedia simulation "Relativity of movement (Einstein's experience)" dealing with the equivalence principle.