- 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