Accelerometer readout
The accelerometer measures acceleration along three axes, including gravity. A phone lying still reads about 9.81 m/s², not zero — which is the single most common surprise.
| Sensor used | Accelerometer |
|---|---|
| Unit | Metres per second squared (m/s²) |
| Useful range | Commonly ±2g to ±16g, hardware dependent |
| Cannot do | Measure speed or distance travelled with any accuracy |
The mechanism, not the marketing.
Inside the chip a tiny mass on springs shifts when the phone accelerates, and that displacement is measured electrically on three perpendicular axes.
Gravity is indistinguishable from acceleration, so it is always included. At rest, the axis pointing down reads roughly 9.81 m/s² and the other two read near zero — which makes the readout a quick way to check orientation.
What a phone cannot do here.
- Deriving speed means integrating acceleration once, and distance means integrating twice. Small errors accumulate quickly through that process, so phone-derived speed drifts badly within seconds. This is why navigation apps use GPS rather than the accelerometer.
- There is a small per-device bias, so a perfectly still phone rarely reads exactly 9.81.
- The chip saturates at its maximum range, so a hard impact clips.
Why does it read 9.8 when the phone is not moving?
Because gravity is a form of acceleration and the sensor cannot separate the two. A reading near 9.81 m/s² on one axis is the phone at rest.
Can I measure how fast I am going?
Not usefully. Speed requires integrating acceleration over time, and the accumulated error makes the result drift within seconds.
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