Why are gears simple machines?
Being stricts, gears are not simple machines, but mechanisms.
Simple machines and mechanisms are, by definition, devices which transform mechanical energy into mechanical energy
On the one hand, simple machines receive as input a single force and give as output a single force. What is their advantage, so? They use mechanical advantage in order to change point of application of that force, its direction, its magnitude...
Some examples of simple machines are:
- Wheel and axle
- Inclined plane
- (And no more, really. These six are the only simple machines which are classically considered since Renaissance)
Mechanisms, on the other hand, are devices which receive a set of input forces and transform it into another set of output forces. Mechanisms are composed by several machines. In that way, gears are really a combination of a wheel and a pulley.
Both simple machines and mechanisms comply with the principle of conservation of energy.