What exactly does the iPhone accelerometer measure?

Question

The apple documentation for UIAcceleration class says,

\"When a device is laying still with its back on a horizontal surface, each acceleration event has

Answer 1

Answerers keep missing the right wording that should set it straight for you... The device is "laying still" only relatively to you. It is actually not laying still at all. The http://en.wikipedia.org/wiki/Centripetal_force of gravity gives it (and you) centripetal acceleration. It is real, it is what keeps you from flying off Earth on a tangent, and it is what the accelerometer dutifully shows. (Earth is nothing special - we rotate about the Sun also etc etc, whose centripetal accelerations are way smaller, but they would be all shown by an accelerometer sensitive enough.)