Students should be able to explain how levers and gears transmit the…

The idea

An explanation needs the mechanism, not just the name. For a lever, the moment of the effort about the pivot equals the moment of the load, so effort multiplied by its distance equals load multiplied by its distance; making the effort distance ten times the load distance lets the effort be a tenth of the load. For gears, the force at the meshing teeth is the same on both wheels, so the moment each one carries is that force multiplied by its own radius. A small gear driving a large one therefore delivers a larger moment, but the large gear turns more slowly and through a smaller angle, which is exactly the low gear a cyclist selects for a hill. Swap them and you get speed instead of turning effect. Meshed gears always rotate in opposite directions.

What earns the marks

• The moments of effort and load about the pivot being equal for a lever.
• The same force acting at the meshing teeth of both gears.
• The moment on each gear given by that force multiplied by the gear's radius.
• A larger gear turning more slowly while carrying a larger moment.

Common mistakes and how to put them right

• Stating that the larger gear turns faster because it is bigger.

• Saying gears change the force at the teeth rather than the moment about each axle.

• Describing the low gear on a bicycle as the one that gives more speed.

• Omitting the reversal of rotation direction between meshed gears.

Exam tips

• Say the same force acts at the teeth; it is the step most answers miss.
• Link large gear to large moment and slow rotation in a single sentence.