Accelerometers are used in many mobile devices (Android included) to track a device's motion and orientation. For example, an Android app that can be used to find a level surface is using the accelerometer to determine the devices orientation.
An accelerometer is a device that measures proper acceleration; proper acceleration is not the same as coordinate acceleration (rate of change of velocity). For example, an accelerometer at rest on the surface of the Earth will measure an acceleration due to Earth's gravity, straight upwards (by definition) of g ≈ 9.81 m/s2. By contrast, accelerometers in free fall (falling toward the center of the Earth at a rate of about 9.81 m/s2) will measure zero.
Accelerometers have …