It occured to me that measuring air resistances R at terminal velocities,
where R=m*g, can be extended to v<v_term, at least in principle. Suppose
that data are collected with a camcorder and that v=f(t) is plotted. Instead
of being a straight line (as when R is negligible) it shows that v is
increasing less rapidly. Thus the values of "a" are less than g. For any v,
Fnet=m*g-R or m*a=m*g-R or R=m*(g-a)
The values of m and g are known before the experiment. The values of "a"
are measured at several v. This provides data for R=f(v). The terminal
velocity case is the limit at which a=0. Trivial? Yes. But worth thinking
about when trying to work at higher v than would otherwise be possible
(that is by using terminal velocities only). I think that the range of
v for which data can be collected does not have to be limited by the
height of a room. The range can be doubled or trippled without going to
those values of "a" which are too difficult to measure accurately. [Some
people claim they can measure g from two velocity points to within 5%
or so. My errors were typically 50% or more. Haw good are measurements
of g from three photogates?]
Ludwik Kowalski