Taken from this stack exchange answer:
http://arduino.stackexchange.com/questions/4223/enable-free-fall-detection-using-arduino[
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"Sitting an accelerometer flatly on a bench will produce a value of 1G in one of the 3 axes.
Free fall of an accelerometer (assuming you haven't reached terminal velocity) should read near zero on all 3 axes.
I am not aware of a mode you can set for this, but if you check the values of the 3 axes and all are close to zero, you are probably in a free fall condition."