How to account for acceleration due to gravity when reading from accelerometer? I'm obtaining readings from an accelerometer, and only care about using these readings to measure the total amount of motion which is occurring. However in order to interpret these readings, I need to somehow account for the contribution to the measured acceleration from gravity. However I'm not sure how to do this without knowing the direction of the gravity vector, which the accelerometer readings alone don't provide. How can I get at this idea of motion which I'm interested in? (Sorry, I'm sure this is a stupid question, but I appreciate the help.) 
 A: As you note (as per General Relativity), an accelerometer cannot separate acceleration from gravity.  Whether there is a solution for you depends on what other information may be available and on what the physics objective is. More details on both the accelerometer (e.g. is it 3-axis, what is its sensitivity?) and your goals would help (e.g. what does "total amount of motion" mean?). 
I suggest looking at the answers to 
How to remove gravity component from accelerometer X, Y readings
and https://stackoverflow.com/questions/3377288/how-to-remove-gravity-factor-from-accelerometer-readings-in-android-3-axis-accel. 
You can just measure g when the accelerometer is not moving and subtract it from your moving readings if the orientation of the accelerometer relative to the local gravitation field never changes and motion is only local, i.e. you are not travelling far enough for the magnitude of g to vary by a measurable amount. It is tough if the accelerometer can rotate and there is no gyroscope or other way to measure rotations. Bandpass filtering can help if the timescale for rotations differs from the timescales for changes in translational motions of interest. Sometimes simply subtracting $|g|$ from the magnitude of the acceleration is adequate.  If you are measuring physical activity, you may want to look at "Separating Movement and Gravity Components in an Acceleration Signal and Implications for the Assessment of Human Daily Physical Activity" (http://dx.doi.org/10.1371/journal.pone.0061691) 
