The way to calculate the sum of the forces in the y direction of an elevator; has been outlined below
If you are in an elevator by itself, then the combined system of you and the elevator could have 3 possible situations which are;
Now, in the vertical y direction, there are three forces namely;
Thus, the sum of forces in the y-direction with the normal force acting on you from the elevator are any of the following;
N = mg; provided that the elevator is at rest or moving at constant velocity.
N = mg + ma; Provided that the elevator has an upward acceleration.
N = mg - ma; Provided that the elevator has a downward acceleration.
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