Hi there, I am reading this old journal and found this formulaes. forces on the seismic mass, expressed in inertial reference coordinates, are given by newton's law as sum of force in x direction = m * x'' sum of force in y direction = m * y'' = Fy = mg the equivalent acceleration of the accelerometer are ax = -F/m = -x'' ay = -F/m = -y'' - g i am rather concerned with the change in sign direction. i vaguely understand the concept of inertial and non inertial ref frame. but i dont know why it will cause a diff to the axial acceleration. hope to hear from someone.