If our model is placed on the earth's surface, the ball will fall onto the Z- wall and will apply a force of 1g on the bottom wall. If the accelerometer is laid on top of a table, this will measure 9.81 m/s2 or -1g. In this instance, the box isn't moving but a reading of -1g on the Z axis is still observed. The pressure that the ball has applied on the wall was caused by the gravitational force. This is meant to illustrate that in essence accelerometer measures force not acceleration. It just happens that acceleration causes an inertial force that is captured by the force detection mechanism of the accelerometer. In this example, if the table is struck from the bottom, an abrupt shock is registered on the Z axis. This reading will look like this: