The mainspring barrel and its gear. Inertial forces from physical accelerations acting on the rotor. Gravitational force from Earth's gravity, also acting on the rotor. Heat from friction, generally negligible but significant once the mainspring is fully wound. Among input sources, inertial forces represent the major portion of the watch's energy supply by an overwhelming margin.
Gravity plays only a minor role on Earth and none in space.
The only exception to this rule is the electrical watch winder , which depends totally on gravity to keep the rotor at the bottom as it turns the watch in a vertical plane about the axis of its hands. That is, the mainspring barrel gear turns at a steady rate with reasonably constant torque as it drives the watch's main gear train and escapement. Of course, the mainspring barrel turns at a constant rate while our rotor's movement is sporadic and uneven. But as long as the mainspring has some capacity left to absorb and store this uneven motion, our system will work reliably providing the long term average input rate matches or exceeds the output rate. We will examine the way a rotor mechanism works in the following section.