TO robert Carlson, Dear Robert:
We fully agree on the FACTS, shis should be clear. This is therefore
a discussion of SEMANTICS, on how we INTERPRET the words, and, by the
way, how we best teach them to students, in order for them to get clear
ideas and avoid confusion.
When students are told about F = m a, two logical situations may
happen. a) They already know what F, m and a mean; in this case this
formula is NOT "just" a definition. b) They already know two of those
magnitudes, in which case this formula will DEFINE the third. I claim
that it is more "natural" to assume that from our sensorial observations
we are already acquainted with acceleration (derived from distance, time and
velocity) and force (from sensorial feelings, and measured by extending
a spring), and on this assumption this is rather the definition of mass.
Of course, when these basic notions are in place, the conclusion
is that there are three physical magnitudes, m, a and Fnet = vector-sum
of all forces acting on the body, and the relation Fnet = m a. In
particular situations it happens often that we know the mass of a body,
we can observe the acceleration, but it may be tricky to find all the
forces acting on the body. This formula then helps us to find out.
If, for example, the problem is in statics (a = 0), then all forces
must be in equilibrium and this leads us to calculate the reactions
of the support and so on. Regards Emilio