Two gears mesh together. Gear A rotates clockwise. Which direction does gear B rotate?

MESHING gears rotate in OPPOSITE directions. Their teeth interlock at the contact point; as one gear pushes its tooth into the other, that contact point on each gear must move in the same linear direction at that instant — but since they're rotating around different axes, this means opposite rotational directions. Common in: car transmissions (gears alternate direction); clock mechanisms; rotating machinery. To keep two gears rotating in the SAME direction, an IDLER GEAR can be placed between them. The idler reverses direction once (from the driver), then reverses again (to the driven gear), resulting in net same direction. Idlers don't change the gear ratio (their teeth count cancels out) but they: (1) Change rotation direction; (2) Bridge a gap between gears that can't mesh directly; (3) Take up slack or stress in chain drives. CHAIN/BELT drives connect gears (sprockets/pulleys) and keep them rotating in the SAME direction (the chain only goes one way around its loop). REVERSED belts (crossed) make them rotate opposite — used in old machinery but uncommon today. PLANETARY GEAR SYSTEMS: sun gear in center, planet gears around it, ring gear around them; complex rotation relationships; used in automotive automatic transmissions and many other applications. RACK AND PINION: a circular pinion gear meshes with a straight rack; converts rotation to linear motion (or vice versa); used in steering systems (turn wheel rotates pinion which slides rack linking to wheels), elevators, machine tools. ASVAB MC commonly tests gear direction prediction in gear trains.