The equivalence principle says, for a small elevator, it is not possible to distinguish between a closed elevator moving at a constant acceleration, and the same amount of gravitational acceleration caused by a massive planet.
If an observer is standing on a planet, the g will be constant. But for an elevator, a constant force is needed to maintain a constant acceleration.
That constant force has to be imparted by some kind of practical mechanism.
Suppose the observer puts a weight on a sensitive scale on the floor of the accelerating elevator, and jumps on the floor. Wouldn't the acceleration momentarily change due to reaction, and weight on the scale would fluctuate.
In order for the weight to not fluctuate, the force has to instantaneously adjust to the reaction, and back, when the observer lands back on the floor.
I am not sure if there is any practical mechanism of constant force that would shield this observation?
I guess only practical way, is that the elevator is very very heavy (same mass as the planet in ideal case). In that case, it will cause its own gravity anyway.
So, is the equivalence principle really practical? Say, for human sized elevator, with simple equipment like a scale? Because the observer can cause the weight to fluctuate inside an elevator, but not (equally) on a planet.
I know, by jumping, the original requisite of the equivalence principle is changed. That is why the question is about practicality.