An accelerometer "is a device that measures proper acceleration; proper acceleration is not the same as coordinate acceleration (rate of change of velocity)
\cite{wikipedia_accelerometer_2017}. For example, an accelerometer at rest on the surface of the Earth will measure the acceleration due to Earth's gravity of g ≈ 9.81 m/s
2 in the up direction. 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". We refer the
Starlino accelerometer guide \cite{starlino_guide_nodate} on describing how to use accelerometer data. To better understand this type of data, It is useful to imagine a box with a ball inside. Next, imagine that each wall is pressure sensitive. If we suddenly move the box to the left (we accelerate it with acceleration 1g=9.8m/s
2)
, the ball will hit the wall X-. We then measure the pressure force that the ball applies to the wall and output a value of -1g on the X axis. Please note that the accelerometer will actually detect a force that is directed in the opposite direction from the acceleration vector. This force is often called Inertial Force or Fictitious Force .