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.
