You can see that if a was equal to g, your downward acceleration is that of gravity, namely the cable has snapped in the elevator, Then you don't feel any weight.

你会发现如果 a 等于 g,你下降的加速度就是重力加速度,也就是电梯的绳索断了,那时你会完全失重

耶鲁公开课 - 基础物理课程节选