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Accelerometer

An accelerometer is a device that measures proper acceleration, also called the four-acceleration. This proper acceleration is associated with the weight of a test mass. For example, an accelerometer on a rocket far from any gravitational influences (assume gravitation is zero) that is accelerating through space due to the force from its engine, will measure the rate of change of the velocity of the rocket relative to any inertial frame of reference, because such changes require application of a (rocket) force that can be felt (as weight), for any mass. However, the proper acceleration measured by an accelerometer is not necessarily the coordinate acceleration (rate of change of velocity), when gravity becomes involved. Gravitation may make these types of acceleration differ. For an example where these types of acceleration differ, an accelerometer will measure a value of g in the upward direction when remaining stationary on the ground, because masses on earth have weight m*g. Such weight is transmitted from the push of the ground, and is not directly caused by gravity, but rather by the mechanical force from the ground, in the same way as the push of the engine in the rocket example. However, there is no change in velocity in this example, as the push from the ground counteracts the acceleration of gravity, which is not felt. By contrast, an accelerometer in gravitational free fall toward the center of the Earth will measure a value of zero acceleration because, even though its velocity is increasing, it is at rest in a frame of reference in which objects are weightless. Like the human body, therefore, an accelerometer is not sensitive to the "acceleration of gravity" per se, and will read zero whenever gravitation provides the only acceleration that acts on the device (or the only "force" on the device). Accelerometers read "zero" on all inertial paths, including f...