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

When you're sitting, gravity is pulling you down to your chair. If it weren't for the chair or ground for that matter, you'd be falling down with acceleration of about 9.8m/s^2. In order for the chair to prevent you from falling down, it has to act with a force in the opposite direction with at least the same value.

The accelometer shows the value of the pulling force and it's a three-dimensional vector. In this case it's directed straight down. And the value given is expressed in G, units of gravity acceleration multiplied by that value.