Accelerometer Physics

An accelerometer measures both acceleration and gravity by sensing the amount of inertial force in a given direction. The accelerometer inside your iOS device is a three-axis accelerometer, meaning that it is capable of detecting either movement or the pull of gravity in three-dimensional space. As a result, you can use the accelerometer to discover not only how the device is currently being held (as autorotation does), but also if it's lying on a table, and even whether it's face down or face up.

Accelerometers give measurements in g-forces (g for gravity), so a value of 1.0 returned by the accelerometer means that 1 g is sensed in a particular direction, as in these examples:

• If the device is being held still with no movement, there will be approximately 1 g of force exerted on it by the pull of the earth.