Part of the problem is how the word weight is defined, which dictates the meaning of the word weightless. Of course, there can be, and are, multiple definitions. As many (if not most) of the posters here are experts (whatever that may mean) in physics or astronomy, they of course have a greater intellectual library from which to draw. Some of us have specific audiences in mind when we answer questions here. I argue that audience must include introductory students who may, indeed certainly, look here for answers.

By definition 1, weight is the nickname for the gravitational force on an object due to Earth. No matter where the object is, it has a well defined weight. During a parabolic flight, the passengers experience essentially the same gravitational attraction to Earth that they experience standing on the ground. The attraction diminishes by only about 10% in low Earth orbit (e.g. aboard the ISS).

Some introductory textbooks incorrectly define weight as the reading on a spring scale when the object is on the scale, and indeed this is a correct operational definition of weight, unless you expect numerical agreement with definition 1 all the time. The scale doesn't measure the object's gravitational weight if you're accelerating; the number depends on the object's state of motion. In general, the spring scale only measures the magnitude of the contact force pressing against the object, and it just so happens that if the object (and the scale) are not accelerating vertically, its weight is numerically equal to the contact force on you from the scale. This approach should not be used, especially in introductory courses because it's dependence on acceleration is confusing to non-experts.

With respect to the sources using definition 1, NASA creates problems for students and interested laypersons by propagating the term weightless. For sources using definition 2, that particular complication doesn't exist, but another arises from the need to involve acceleration. Which one is better is not dictated by the most frequently used one, as that constitutes a fallacious appeal to the populous and perhaps also a fallacious appeal to authority. Which one is better should be dictated by which causes the less confusion and fewest misconceptions.

The answer depends on how the word weight is defined, which then dictates the meaning of the word weightless. Of course, there can be, and are, multiple definitions. As many (if not most) of the posters here are experts (whatever that may mean) in physics or astronomy, they of course have a greater intellectual library from which to draw. Some of us have specific audiences in mind when we answer questions here. I argue that audience must include introductory students who may, indeed certainly, look here for answers.

By definition 1 (which was the definition presented to me in my undergraduate and graduate courses at two universities), weight is the nickname for the gravitational force on an object due to Earth. No matter where the object is, it has a well defined weight. During a parabolic flight, the passengers experience essentially the same gravitational attraction to Earth that they experience standing on the ground. The attraction diminishes by only about 10% in low Earth orbit (e.g. aboard the ISS).

Some introductory textbooks define weight as the reading on a spring scale when the object is on the scale, and indeed this is a correct operational definition of weight, unless you expect numerical agreement with definition 1 all the time. The scale doesn't measure the object's gravitational weight if you're accelerating; the number depends on the object's state of motion. In general, the spring scale only measures the magnitude of the contact force pressing against the object, and it just so happens that if the object (and the scale) are not accelerating vertically, its weight is numerically equal to the contact force on you from the scale. This approach should not be used, especially in introductory courses because it's dependence on acceleration is confusing to non-experts.

A potential consequence of definition 2 is that without a contact force, one must then deal with questions about how and why things float inside spacecraft in low Earth orbit, and this issue is inevitable among even sophisticated students. Yes, the experts among us know that there's no floating involved and that the craft and occupants are merely free falling.

With respect to the sources using definition 1, NASA creates problems for students and interested laypersons by propagating the term weightless. For sources using definition 2, that particular complication doesn't exist, but another arises from the need to involve acceleration. Which one is better is not dictated by the most frequently used one, as that constitutes a fallacious appeal to the populous and perhaps also a fallacious appeal to authority. Which one is better should be dictated by which causes the less confusion and fewest misconceptions, a criterion easily measurable in the classroom.

Regardless of which definition of weight one uses, I hope we can agree that if the passengers and aircraft are all in a state of free fall, there is nevertheless a gravitational force due to Earth on them, and that this force exists no matter what, and that no contact force (aside from casually bumping into the aircraft's interior or another passenger) exists between aircraft and passenger.

By definition, weight is the nickname for the gravitational force on an object due to Earth. No matter where the object is, it has a well defined weight. During a parabolic flight, the passengers experience essentially the same gravitational attraction to Earth that they experience standing on the ground. The attraction diminishes by only about 10% in low Earth orbit (e.g. aboard the ISS).

What most laymen mean by weightlessness is really contactforcelessness, which is a bit of a made up word, but it accurately reflects the underlying physics. When the floor or seat that supports us is in free fall along with us, we don't sense its presence and thus we are tricked into thinking we are weightless. In actuality, what we sense is the absence of a supporting contact force on us. So, when the aircraft goes into free fall, the astronauts do too, and lose contact with the aircraft's floor. Understand that they are not floating! They are merely in free fall, but so is the aircraft! The gravitational force Earth exerts on them when on the ground still acts on them! They couldn't experience free fall if it didn't. The gravitational force Earth exerts on the aircraft when on the ground still acts on it too! It couldn't go into free fall if it didn't.

Now, some introductory textbooks incorrectly define your weight as the reading on a spring scale when you're standing on it. This is problematic for several reasons. First, the scale doesn't measure your weight. It only measures the contact force pressing against you, and it just so happens that if you (and the scale) are not accelerating vertically, your weight is numerically equal to the contact force on you from the scale. Second, if you accelerate upward, the reading on the scale will change (you should be able to figure out how it will change) and thus your weight will change even though the Earth's gravitational attraction on you doesn't change significantly. Third, if you accelerate downward, a similar thing happens. This approach should not be used, especially in introductory courses because it's confusing and doesn't accurately reflect the underlying physics.

One more thing about weight. Weight is a gravitational force, and no one has ever felt (in the traditional sense) such a force. We feel floors, seats, and other people pressing against us, but these are all contact forces, not gravitational forces. In certain circumstances, any of these forces may have the same magnitude as our weight, but not in general. Einstein would say this is perfectly logical, because gravitational "forces" don't actually exist in the first place! He would argue that gravitational attraction is caused by spacetime geometry, not some mysterious force. Note that "force" is a concept in Newtonian mechanics. Also, note here that I am certainly NOT arguing that gravitational attractions doesn't exist, just that there are several ways of describing it and the concept of force is but one way. Spacetime curvature and the principal of stationary action are other ways to describe the exact same thing.

I'll close by saying that NASA does a huge disservice to students and interested laypersons by propagating the term "weightless." It's a word that needs to be eliminated from the physics vocabulary.

Part of the problem is how the word weight is defined, which dictates the meaning of the word weightless. Of course, there can be, and are, multiple definitions. As many (if not most) of the posters here are experts (whatever that may mean) in physics or astronomy, they of course have a greater intellectual library from which to draw. Some of us have specific audiences in mind when we answer questions here. I argue that audience must include introductory students who may, indeed certainly, look here for answers.

In introductory physics literature, weight is usually defined as

1. the force on an object due to gravitational interaction with Earth

or

2. the contact force on an object, as measured by a spring scale, due to gravitational interaction with Earth.

By definition 1, weight is the nickname for the gravitational force on an object due to Earth. No matter where the object is, it has a well defined weight. During a parabolic flight, the passengers experience essentially the same gravitational attraction to Earth that they experience standing on the ground. The attraction diminishes by only about 10% in low Earth orbit (e.g. aboard the ISS).

A potential consequence of definition 1 is that if the object on Mars, does it still have weight? It does, but that weight still involves interaction with Earth. There is also obviously a gravitational interaction with Mars, but if we adhere to definition 1 we can't name that force weight.

What most laymen and students mean by weightlessness is contactforcelessness, which is a bit of a made up word, but it accurately reflects the underlying physics. When the floor or seat that supports us is in free fall along with us, we don't sense its presence and thus we are tricked into thinking we are weightless. In actuality, what we sense is the absence of a supporting contact force on us. So, when the aircraft goes into free fall, the astronauts do too, and lose contact with the aircraft's floor. Understand that they are not floating. They are merely in free fall, but so is the aircraft. The gravitational force Earth exerts on them when on the ground still acts on them. They couldn't experience free fall if it didn't. The gravitational force Earth exerts on the aircraft when on the ground still acts on it too. It couldn't go into free fall if it didn't.

Some introductory textbooks incorrectly define weight as the reading on a spring scale when the object is on the scale, and indeed this is a correct operational definition of weight, unless you expect numerical agreement with definition 1 all the time. The scale doesn't measure the object's gravitational weight if you're accelerating; the number depends on the object's state of motion. In general, the spring scale only measures the magnitude of the contact force pressing against the object, and it just so happens that if the object (and the scale) are not accelerating vertically, its weight is numerically equal to the contact force on you from the scale. This approach should not be used, especially in introductory courses because it's dependence on acceleration is confusing to non-experts.

Ultimately, the concept of gravitational interaction underlies this issue, and no one has ever directly felt (in the traditional sense) such a force. We feel floors, seats, and other people pressing against us, but these are all contact forces, not gravitational forces. In certain circumstances, any of these forces may have the same magnitude as our weight, but not in general. Einstein would say this is perfectly logical, because gravitational forces don't actually exist in the first place. He would argue that gravitational attraction is caused by spacetime geometry, not some mysterious force. Note that force is a concept originating in Newtonian mechanics, but isn't necessary in all explanations of gravitational attraction and it thus of limited usefulness.

With respect to the sources using definition 1, NASA creates problems for students and interested laypersons by propagating the term weightless. For sources using definition 2, that particular complication doesn't exist, but another arises from the need to involve acceleration. Which one is better is not dictated by the most frequently used one, as that constitutes a fallacious appeal to the populous and perhaps also a fallacious appeal to authority. Which one is better should be dictated by which causes the less confusion and fewest misconceptions.