device that measures proper acceleration (opens in new tab)

An accelerometer is a device that measures the proper acceleration of an object.[1] Proper acceleration is the acceleration (the rate of change of velocity) of the object relative to an observer who is in free fall (that is, relative to an inertial frame of reference).[2] Proper acceleration is different from coordinate acceleration, which is acceleration with respect to a given coordinate system, which may or may not be accelerating. For example, an accelerometer at rest on the surface of th...