Hi!
The question is: what is better? Statics or kinematics first?
(The issue is NOT if I "observe" forces and accelerations with my
eyes or with my belly!! It also is NOT a question of very sophisticated
philosophy - according to Heisenberg one even cannot measure both
position and velocity of a particle exactly!) It is, indeed, a matter
of practicality for a beginning students.
Since one cannot learn everything at the same time, one MUST go
step by step, trying to make each step clearly understandable. From
the simple to the complex.
Both ways (statics or kinematics first) CAN BE DONE meaningfully.
If kinematics is first, we start with the notion of POSITION and
TIME. Bodies are points, no extension nor mass. No forces to make them move,
they just move. The concept of VELOCITY should be analyzed, which is
not difficult. Then comes ACCELERATION, which is more difficult,
particularly since velocity is a vector. When this is accomplished
we can move into dynamics, but we need FORCES.
If statics is first, we start with the notion of FORCE. There is no
problem with that, any elementary school child knows how to pull or
push an object. That forces are just the source of acceleration is contrary to
everydays experience: I push the piano but nothing moves (I need at
least two people to make it move!). So we discuss vectors and
equilibrium. The state of rest is obvious: nothing moves. Here we
interject that we will see later that this is equally valid for a
frame with uniform motion, for example the Earth or in a moving car,
railway or plane.
The choice between the two ways (stat - kinem) is up to the
teacher. I tend to believe that statics first is easier for the
student. In statics we ignore the TIME. Just geometry and forces.
When we then start kinematics, the student is already familiar with
vectors. And after kinematics we go to dynamics which ties both
together, and we have no problem talking about the NET force (the
vector sum of all forces) acting on a body and so on.
Regards Emilio