circular component rotating on an axle
one of six simple machines, alongside lever, wedge, screw, pulley, inclined plane
invented circa 3500 BCE in Mesopotamia, first for pottery then transport
applications: vehicles, gears, turbines, flywheels, pump impellers
converts linear force to rotational motion and back
foundational to nearly all mechanical energy transmission
the engine is a wheel-dependent system: crankshaft, camshaft, drivetrain
scaling from nanometer gears to wind turbine rotors