Weightlessness is the experience (by people and objects) during free-fall, of
having no apparent weight. Although the term 'zero gravity' is often used as a
synonym, weightlessness in orbit is not the result of gravity itself being
eliminated or even reduced significantly (in fact, the acceleration due to
gravity at an altitude of 100 km is only 3% less than at the earth's surface — a
person at rest at that altitude would accelerate to earth at a familiar rate).
Weightlessness (roughly speaking) occurs when a body (e.g. a person) is: falling
freely; in orbit; in outer space (far from a planet, star, or other massive
body); in an airplane following a particular parabolic flight path (e.g. the
"Vomit Comet"); or one of several other (even more unusual) frames of reference.
More generally, weightlessness occurs when a person (or object) is subject (at most) to the single force of gravity (or is not acted upon by any accelerating force), vs. the far more typical (in human experience) cases in which an equal/opposite force is acting — such as:
1. standing on the ground, sitting in a chair on the ground, etc. (gravity is countered by the reaction force of the ground);
2. flying in a plane (gravity is countered by the lift the wings provide)
3. atmospheric reentry, use of a parachute: atmospheric drag decelerates the vehicle;
4. during an orbital maneuver in a spacecraft, or during the launch phase: the rocket provides thrust.
(The principal difference is that gravity acts directly on a person and/or other bodies, just like on the vehicle's mass — whereas forces like atmospheric drag and thrust act only on the vehicle body itself (and thus only secondarily, through the vehicle, on the person). In the first case the person and the vehicle floor are not 'pushed' towards one another; in the other cases, the force is transmitted through the vehicle's structure to the person and/or contents.)
What humans perceive as "weight" is not actually the force of gravity pulling us towards the ground (actually, towards the center of the Earth — although this is the technical definition of "weight"). What we feel as "weight", is actually the normal reaction force of the ground (or whatever surface we are supported by) "pushing" upwards against us to counteract gravity's downward pull — that is: the "apparent weight". (In the remainder of this article, the term 'weight', without 'apparent', is used in this sense.) While this is not always intuitive, imagine the floor dropping out from under you: without it, you'd be falling — and experiencing weightlessness. It's the floor (or ground — whatever), supporting you against gravity's pull — and which keeps you from falling to the center of the Earth — that creates the sensation of "weight".
example: a wood block in a container in free-fall "experiences" weightlessness.
This is because there is no force from the container's bottom on the block,
against the pull of gravity, as both the container and the block are being
pulled down with the same acceleration. When the container is at rest on the
ground, however, the force of gravity pulling downwards on the block is exactly
matched (in the opposite direction, and by the same amount) by the support of
the bottom of the container.
Because the block is a solid, each horizontal cross section of the block experiences not only the force due to gravity on it, but also the weight of whatever portion of the block is above it. (In the case of an object, or portion thereof, which is not supported from below, but suspended from above, a 'negative pressure', or tension gradient exists. It occurs because each cross section of a hanging object, a rope for instance, must support the weight of every piece below it.) Part of feeling "weight", then, is actually experiencing such a pressure/tension gradient within one's own body parts (e.g.: while standing on one foot, the foot on the ground would feel the pressure of the entire body's weight, whereas the other leg and both arms would feel/be subjected to the tension gradients of their own weight being pulled down against their sockets).
In free-fall, a person or object experiences no measurable (or apparent) weight because all parts of the object are accelerating uniformly (any variations in acceleration due to tidal forces being imperceptible)..
Ours sponsors are Cruise and Stay, Cruise and Stay,