Alpha, the fact that participants are considering the gravity issue suggests that the weighing method is something other than a balance with a standard 45g weight. Are you quite sure that this is the case? Given the essentially scientific nature of the event, it is incongruous.

Alpha: "How would I calculate the loss of moisture content?"

I don't have a clue. That said, I do suspect that under normal atmospheric conditions changes are only significant over long periods of time - weeks at least - because (a) the permeability of balsa wood may be low, except in the case of pressure impregnation (and if you oil painted the whole car, it would be effectively sealed) (b) the mean moisture gradient over time is unlikely to be very great. As I said, I don't know, but they are factors to think about.

Paul: "the m will decrease due to the lower air pressure."

Sorry to be dense (! smile ), but could you explain how decreased air pressure causes decreased mass? The volume and density may change, but not the mass.

"Time is what prevents everything from happening at once" - John Wheeler