Final answer:
The group of gears that turn the back wheel of a bicycle is part of the drive train system, functioning as a wheel and axle. Changing gears alters the mechanical advantage and the torque applied to the wheel, impacting its angular momentum.
Step-by-step explanation:
The group of gears that turn the back wheel of a bicycle is considered part of the bicycle's drive train system, which utilizes principles of physics to transmit power from the rider to the wheel. The gears work in conjunction with the chain to create a wheel and axle system, which is a form of simple machine. When cyclists change gears, they are effectively changing the radius of the axle in the gear system, altering the mechanical advantage (MA) and the torque applied to the rear wheel. This torque creates a force that propels the bicycle forward and can also affect the angular momentum of the bicycle wheel.