How can a phone sense movement with a tiny moving mass?

Inside a phone’s accelerometer is a tiny mechanical structure made using chip-manufacturing techniques. One common design suspends a small proof mass on flexible supports. Fixed electrodes sit close to parts attached to that mass.

When the housing accelerates, the mass's inertia and the supports' forces produce a small relative displacement. The spaces between movable and fixed electrodes change. Those surfaces form capacitors: their capacitance describes how much separated charge they hold for a given voltage. Moving facing surfaces closer together or farther apart changes that relationship. The sensing circuit converts the tiny mechanical displacement into a measurable electrical change.

After processing and calibration, the device reports values for one or more directions. A phone can combine readings from several axes to help respond to tilting or shaking. A phone at rest can still show a gravity-related reading, and tilting changes how that reading appears across the axes.

The sensor is not a miniature map or speedometer. Position estimates would require additional calculations and other information, and small errors can accumulate. Gyroscopes provide a different measurement related to rotation. A phone combines these clues because one sensor alone does not describe every aspect of motion.