How does my smartphone know which way is up?

A: Your smartphone knows which way is up due to its accelerometer. It’s a chip whose parts shift slightly when the phone accelerates or when gravity acts along a particular direction. The phone’s software reads the accelerometer’s measurements along the x, y, and z directions and figures out which side is pointing towards the earth. This is how your phone knows which way to turn the screen when you rotate it.

Phones also have a gyroscope, which measures how fast the phone is rotating. This is a microelectromechanical sensor with a small vibrating mass on a chip. The phone drives this mass to vibrate back and forth at a fixed frequency. When the phone rotates, the vibrating mass is deflected sideways due to the Coriolis effect. If the chip is vibrating in one direction and the phone rotates around a particular axis, the mass is pushed slightly in a direction perpendicular to the vibration.

The sensor has microscopic springs and electrodes to detect this sideways motion and convert it into electric signals, from which the phone estimates the angular velocity, how fast the phone is turning and in which direction.