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>how does a plane know it's stalling if the pitot tubes are frozen?

I think it can easily do that by measuring the G force similar to an accelerometer on iPhone.

No. There is, not surprisingly, another sensor in the airflow for that.

http://www.lindenhillimports.com/images/photof48.jpg

Accelerometers measure acceleration. You won't feel any unusual G forces for any constant velocity, whether it's climbing or descending.

Yes you're right...Stall is not a free fall but a sudden reduction in lift which can occur for instance when the angle of attack increases beyond a certain value.

It's actually not a sudden reduction in lift either. Even in a stall, lift equals gravity. It's just that the normal relationship between angle of attack and lift is reversed to that higher angle of attack lowers lift rather than increasing it. This makes it inherently unstable and difficult to control, but there is in principle nothing preventing you from flying a stalled airplane all day.

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