The point is force A is not zero,
For the same amplitude in acceleration, force A is smaller than force B. Force B accounts for work against gravity also. When acceleration is zero, force B is only working against gravity. On a spring balance, inertial mass disappears. The mass weighed is only gravitational mass.
Huge implication if you are trying to launch vertically (with a vertical gravitational component).
F=mg
Given any field, what do you call m, that property of the object that divides the force in the field to give acceleration g. It is the same m when energies (potential and kinetic) is in focus.
Good night.
