Wizo
I think you are in real danger of tying yourself in knots over this.
In order that an aircraft maintain altitude it must produce lift equal to its' apparent weight, not just the force due to gravity.
In order to maintain its existing vertical velocity, the vertical components of all forces acting on the aircraft must (by vector addition) sum zero.
Taking a few assumptions: slowish aircraft (i.e. not a spacecraft), lowish angle of attack (i.e. vertical thrust component negligable), in level flight to start off with.
In this situation we can say that the vertical component of lift must be equal and opposite to the weight.
In terms of predicting the result of any physical interaction a bodies apparent weight is actually more important than it's actual weight.
Well, for calculating interactions between objects in the system, contact forces are obviously important. But for external 'wheres-the-damn-thing-going' kind of calculations there is a lot of virtue in recognising that the aircraft weight is constant (in the short term) and has a fixed orientation.
Objects in free-fall act exactley as if they were not in a gravity field.
Agreed, its just newton 1,2 and 3
Objects subject to both gravity and a normal acceleration act exactley as if they weighed more.
Their
apparent weight has increased, their actual weight is unchanges (assuming no change in gravitational field strength.)
Weight is measured with a scale,
Apparent weight is measured with a scale
and in both cases, that measurment would be different to 1g, and there would be no way for an observer to tell the difference
It is true (and crucially important) that an observer inside can not tell the difference. However, an external observer may well wish to quantify between them (weight and an acceleration caused by an unbalanced contact force) in order to relate the situation to his/her own frame of reference.
between force due to gravity and force (or lack of it) due to acceleration.
The danger in this statement is that people can get in a muddle about Newton 2. Forces cause accelerations, not vice versa. I get very concerned when someone starts talking about a force due to an acceleration.
F=MA tells us what force must have been present in order to cause the mass M to undergo acceleration A.
It does
not imply that force F exists because object M accelerated.
In summary, I'm with Checkboard. Weight is a defined term. Its constant (in this context). Apparent weight is also a defined term, its variable (in this context). Use them interchangably at your peril.
pb