I was reading this on reddit:
If you were in an elevator accelerating upwards which, you might
experience a force of +2g. And if the elevator was accelerating
downwards very quickly, you might actually feel an upwards force of
-0.5g. That's what a negative g-force is, when it feels like you are falling up.
So I understand that the case scenario when an elevator is accelerating upwards, the net force on the person is in the up (positive) direction so the force applied by the person in reaction is in the down (negative) direction, which is positive g-force.
But I don't at all understand how you will feel an upwards force of -0.5g when an elevator is accelerating downwards. Because when in an elevator, accelerating downwards at theoretically $4.9m/s^2$, the force normal will still be upwards (as it's preventing free fall), but will be less than if there was no acceleration (less weight). But the reaction force therefore is downwards. So this is still just like the scenario when the elevator is accelerating upwards!
Or am I misreading this? Does this person actually mean if the elevator is somehow accelerating downwards at 1.5 times the acceleration of gravity? In that case, I don't see how this would make any sense.
Best Answer
When we say you are experiencing an acceleration this means something must be exerting a force on you, because force and acceleration are related by Newton's second law. In a stationary elevator it is the floor of the accelerator that exerts an upwards force on you, and this force is just $mg$ giving you an acceleration of $g$.
If the elevator is accelerating downwards, for example $-0.5g$, the force exerted on you by the floor of the elevator is decreased and your total acceleration is decreased (in this case to $+0.5g$). If the downward acceleration of the elevator is $-1g$ then the force on you decreases to zero and you become weightless i.e. your acceleration is zero $g$.
If the acceleration of the elevator becomes greater (more negative) than $-1g$ you will find yourself standing on the roof of the elevator so it is now the roof that exerts a net downwards force on you. This is how your acceleration can become negative. Instead of the elevator floor accelerating you upwards the elevator roof accelerates you downwards.