How does the iphone's accelerometera works?

Question

how does the accelometer still working even when the iphone is shut down?
when I start the iphone after complete shut down and hold it in front of my face of knows what angle is this. how? accelerometer measure movement...

Answer 1

unless the iphone have another battery that keep measure the iphone movement even when it shut down, and then when I start the IPhone again it's know what he's position.
The iPhone does not remember its position. When it's turned on, the accelerometer detects the force of gravity, so can calculate how much the phone is tilted. This tilt reading, combined with the magnetic compass reading, can determine the phones orientation. The GPS can give the phone's location. None of this requires knowing anything that happened to the phone while power was off.
This is unlike an inertial guidance system, which has to continuously monitor rotation and acceleration to calculate how far it has moved from a starting position. An inertial system, would use too much power, and be too large and expensive, for a hand-held device. They are used for systems that have to work without outside signals, such as radio beacons or GPS. Inertial guidance systems are used on submarines, for some underground position measurements (such as mining and oil well drilling), and for plane and missile systems that have to work when GPS may be unreliable due to jamming or satellite destruction.