newtonian mechanics - Reduced-gravity flight (A300 0-G): what is actually reduced?

I doubt I can hide from the effects of gravity when I'm close to Earth. We usually don't question this biker is fully experimenting gravity:

Source.

But with the well-known "reduced gravity" ballistic flight path provided by the Zero-G A300, it's common to talk about micro-gravity, or reduced gravity, or 0-G.

Source.

It seems to me that the gravity in the A300 is not reduced at all, it's still the same than when flying straight and level. Exactly like the gravity hasn't changed for the stunt biker.

I also often read that the weight appears null, due to the free fall. But again I don't understand... According to Merriam-Webster:

Weight: The force with which a body is attracted toward the earth or a celestial body by gravitation and which is equal to the product of the mass and the local gravitational acceleration.

And that's what I learned: $w = mg$, and because mass and gravity acceleration haven't changed, then it seems weight hasn't changed either.

If gravity and weight haven't changed, then what is reduced actually in so-called reduced gravity? What does really happen to the aircraft and its occupants?

The only thing that I can see as new in free fall is that the different parts of a body (or the of the aircraft) are not anymore ordered from top to bottom by their weight. For example air will not be on top of fuel in the aircraft tanks, and blood will not be moved towards our feet without an action.

Answer

The aircraft gives you a protected area in which you can experience free fall without any drag or other significant forces having a large effect on your body. This is essentially identical to what an astronaut experiences on the International Space Station, only with the drawback that the semi-minor axis of your orbit is so small that it has two intersection points with the surface of the Earth. Obviously, the pilot pulls up before that becomes a problem.

If you are inside the aircraft, your intuitive frame of reference is what you see around you - the interior of the aircraft. Relative to the aircraft, the acceleration is very close to zero because it falls with you and uses the engines to cancel out the drag it experiences. Relative to the surface, your acceleration is around 9.81 $\frac{m}{s^2}$, as expected during free fall. This is not proper acceleration, so it is impossible to sense or measure (unless you measure your change of speed relative to the Earth).

The term "zero g" is nevertheless accurate if it is understood as describing the g-forces. The definition of g-force from Wikipedia is:

The g-force acceleration experienced by an object is due to the vector sum of all non-gravitational and non-electromagnetic forces acting on an object's freedom to move.

So g-force, on the surface of the Earth, is actually the combination of the acceleration caused by gravity and the ground preventing you from freely moving as this acceleration would dictate. If the surface of the Eearth, or any other object in your way, is temporarily (mostly) removed, the g-force acting on your body is approximately zero. This is commonly described as a micro-$g$ environment or microgravity to highlight the fact that

• There are sill some small forces acting upon an object because all micro-$g$ environments have imperfections.


• Gravity is still accelerating an object in $\mu g$. It's just not perceived because every part of the object is accelerating at the same rate resulting in no internal forces (i.e. there is no proper acceleration).