A short rant about the Kate Upton "vomit comet" photo shoot...

[jberryhill]

147. The point is

Saying "the international definition of weight is the Newton and it is mg" does not have any relevance to the usage of the word "weightless" as it is indeed used by NASA and everyone else to refer to a condition of free fall where there are no external opposing forces countering the action of gravity on a body.

"Weight" is not intrinsic. When a body of mass m is sitting on a table, it is - yes indeed - exerting a force of mg on the table, and the table is exerting a counteracting force of mg on the body. And, sure enough, both of those forces are measured in Newtons. The Newton is a unit of FORCE, and is the gravitational case of F=ma, which is the inertial force equation. The "weight", in Newtons, tells you what those equal and opposite forces are when that body is sitting on the table.

Now, the table and the mass are in a falling elevator, an elliptical airplane ride, or the space station. The body is no longer exerting any force, in Newtons, on the table, and vice versa. In that context, there is nothing, nada, zero that you are going to measure in Newtons between that body and that table. The body has become weightless - you cannot measure its weight (i.e. the force, in Newtons, it would exert on the table) because it is not exerting any weight on the table, and the table is not exerting a counteracting force back. Because - in the weightless, free-fall condition, there are no forces being exerted between them.

That is why it is correct to refer to the Newton as a measure of "weight" because, more fundamentally, the Newton is a measure of FORCE, and it falls right out of the Newtonian inertial equation of F=ma.

"Weightlessness" is the condition of a body upon which no external forces are acting. And by that we don't mean "it is falling under the influence of gravity, so gravity is acting on it" - we mean "there is nothing imposing a FORCE on the body to counteract gravity".

It is not a misnomer.