Ok guys, I think the pre-GCSE, pre-60's class are getting confused about units of measurement here.
The overiding principle is that mass != weight. An aircraft with mass of 800kg does not have a weight of 800kg. To demonstrate this I will make up a whole new set of measurements so that we are on a clean footing.
Thus I have am using the PP standards of bags of sugar for mass, football pitches per hour for velocity.
Thus
I don't think there's any disagreement in this ?
Mass of aircraft = x bs (x bags of sugar)
Everybody agree so far ?
Weight of aircraft = x . a
Everybody still in agreement ?
I am presuming we are all happy with the concept of gravity acting downwards, and will define it thus
Gravity = [0.0 -y 0.0]
I.e. a 3d vector that shows gravity acting downards.
Therefore
Weight = x . [0.0 -y 0.0]
Thus
Weight = [0.0 -xy 0.0]
Hence it can be demonstrated that weight and mass are not equal, the big stumbling block being that mass is a scalar, acceleration and thus weight is a vector.
Under certain circumstances, i.e. a gravitational field with strength 1, the modules of weight and mass may be numerically the same, but they are still not the same given the fundamental problem of mass being a scalar and accleration or weight being a vector.
QED (Quite easily done)
Ahh, "Principles of flight" by Jeremy Pratt - the thread that keeps on giving