Thanks again for the reply. You are fast! (good for me, quite stressed by now :P)
I understand that if I have a 2 axis accelerometer, if I have it in a plain surface, is the same not moving that falling plain. Anyway, I am planing to put my sensor in 3D, but, tipycally it will be with the y axis pointing to the floor and the x asis moving in the horizontal plain. I am not sure to have explain myself properly. the problem is, as you said, that is noot moving in a plain, but in the space, but having these considerations, can anything be done?
I will explain in 2 lines little bit of what I want to achieve, so it may clear something: I want tsomeone to carry the board with the seonsor, and detect when the person falls, so I though a good idea could be to detect sudden changes in (motion) acceleration. Thank you again!