Piezoelectric materials produce a voltage difference between sides, when subjected to a stress (pressure). Pushing on them produces a voltage difference between sides. Actually they are more sensitive in the resistance mode, where stress produces a large change in resistance. They may be used to measure atmospheric pressure differences as a common microphone or as a pressure sensor. With a mass attached, they may be used to detect accelerations, like the detectors in cell phones that keep projected images upright in the Earth’s gravitational field.