aikilouis

In the case of the screwjack, in the F=ma formula the only m that is relevant is the car's mass (actually a fraction of it, because the screwjack isn't supposed to lift the whole car). Erick explained very clearly how the operator exerts his force through the screwjack's mechanism (itself standing on the ground, hence using the earth's reaction force) to move the car and affect its position. The operator's mass isn't relevant here, what matters is that he exerts force, one way or the other.

If we apply the same model to the teacher's test, the car is the tester, the teacher is both the operator (source of initial force) and the screwjack (whose purpose is to transform the existing forces = gravity, earth's reaction, operator's force).

I'm reading Tohei sensei's Book of Ki, and the way he describes hi famous 4 principles is actually about optimisation of the body's inner tensions (used to sustain the body's structure in static and dynamic situations) as well as the combination of forces that it cannot escape (gravity and the earth's reaction).