An
accelerometer is a device that measures
proper acceleration ("
g-force"). 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
g= 9.81 m/s2 straight upwards. By contrast, accelerometers in
free fall (falling toward the center of the Earth at a rate of about 9.81 m/s
2) will measure zero.