Answered myself :

Since the acceleration measured by the sensor includes the force that gravity is exerting on it—and since we know that the acceleration of gravity (barring any other forces applied to an object) is 9.8m/s/s—this lets the sensor determine which direction its facing. While at rest, the sensor will return no acceleration forces on two of its axes, but one axis should be feeling acceleration of 9.8m/s/s. This is also sometimes referred to as 1 “g” of force.

My android device Accelerometer with 3 axis never shows 1g when its placed on table ,facing screen upward , Fall value is the magnitude of all three axis value using above formula .

2022-04-05 09:17:02.581 13223-13223/org.pjsip.pjsua2.app E/x: 0.0503082275390625
2022-04-05 09:17:02.581 13223-13223/org.pjsip.pjsua2.app E/y: -0.03803062438964844
2022-04-05 09:17:02.581 13223-13223/org.pjsip.pjsua2.app E/z: 9.877481460571289
2022-04-05 09:17:02.595 13223-13223/org.pjsip.pjsua2.app E/FALL: 9.9
2022-04-05 09:17:02.595 13223-13223/org.pjsip.pjsua2.app E/x: 0.07606124877929688
2022-04-05 09:17:02.595 13223-13223/org.pjsip.pjsua2.app E/y: -0.060489654541015625
2022-04-05 09:17:02.595 13223-13223/org.pjsip.pjsua2.app E/z: 9.892154693603516
2022-04-05 09:17:02.611 13223-13223/org.pjsip.pjsua2.app E/FALL: 9.9
2022-04-05 09:17:02.611 13223-13223/org.pjsip.pjsua2.app E/x: 0.06258583068847656
2022-04-05 09:17:02.611 13223-13223/org.pjsip.pjsua2.app E/y: -0.08264923095703125
2022-04-05 09:17:02.611 13223-13223/org.pjsip.pjsua2.app E/z: 9.901138305664062
2022-04-05 09:17:02.626 13223-13223/org.pjsip.pjsua2.app E/FALL: 9.9
2022-04-05 09:17:02.627 13223-13223/org.pjsip.pjsua2.app E/x: 0.0886383056640625
2022-04-05 09:17:02.627 13223-13223/org.pjsip.pjsua2.app E/y: -0.07396507263183594
2022-04-05 09:17:02.627 13223-13223/org.pjsip.pjsua2.app E/z: 9.863407135009766
2022-04-05 09:17:02.658 13223-13223/org.pjsip.pjsua2.app E/FALL: 9.9
2022-04-05 09:17:02.658 13223-13223/org.pjsip.pjsua2.app E/x: 0.09223175048828125
2022-04-05 09:17:02.658 13223-13223/org.pjsip.pjsua2.app E/y: -0.03803062438964844
2022-04-05 09:17:02.658 13223-13223/org.pjsip.pjsua2.app E/z: 9.85472297668457
2022-04-05 09:17:02.673 13223-13223/org.pjsip.pjsua2.app E/FALL: 9.9
2022-04-05 09:17:02.673 13223-13223/org.pjsip.pjsua2.app E/x: 0.07336616516113281
2022-04-05 09:17:02.673 13223-13223/org.pjsip.pjsua2.app E/y: -0.04791259765625
2022-04-05 09:17:02.674 13223-13223/org.pjsip.pjsua2.app E/z: 9.853225708007812
2022-04-05 09:17:02.689 13223-13223/org.pjsip.pjsua2.app E/FALL: 9.9
2022-04-05 09:17:02.690 13223-13223/org.pjsip.pjsua2.app E/x: 0.08234977722167969
2022-04-05 09:17:02.690 13223-13223/org.pjsip.pjsua2.app E/y: -0.07516288757324219
2022-04-05 09:17:02.690 13223-13223/org.pjsip.pjsua2.app E/z: 9.873289108276367
2022-04-05 09:17:02.720 13223-13223/org.pjsip.pjsua2.app E/FALL: 9.9

I wish to know when should I declare an smart phone static by reading its 3 axis accelerometer reading.

I have notice that modulo of all 3 axis gives 9.8 when ever smart phone is static. what does it say  , if the phone is not moving shouldn't it show 0 instead of 9.8.

I wish to know when should I declare an smart phone static by reading its 3 axis accelerometer reading.

I have notice that modulo of all 3 axes gives 9.8 when ever smart phone is static. What does it say, if the phone is not moving shouldn't it show 0 instead of 9.8.

$$ |a| = \sqrt{x^2+y^2+z^2} $$