Gears (Wheel and Axle)


A gear is a wheel with teeth along the outer edge that rotates on an axle. By itself a gear can’t do much.  But in combination with other gears, gear pairs can change the direction, speed, and torque (rotational force) of rotation.

One gear will turn in one direction and the other gear in the opposite direction.

The large gear on the left will turn more slowly than the smaller gear on the right.  The large gear also can handle more torque (rotational force) than the small gear does.

If you have a bike that can shift gears, you may already know a great deal about this matter. If you are going fast on a level or downhill road, your feet are turning a large gear, which is making a small gear (connected to the rear wheel) turn much faster.  It is relatively hard to pedal.

As you went downhill, your feet would be turning the pedal gear slowly, but the gear on the back wheel would be turning very fast.  Also, it feels hard to pedal.

When you are going uphill, you shift gears so your feet are turning a small gear at a fast rate, and this is connected to a larger gear that is turning more slowly.  It is relatively easier to pedal.  The principle of the lever applies: You use less effort but apply it over a greater distance - the longer circumference of the larger gear; more pedaling is required to go the same distance compared to above.

As you went uphill, your feet would be turning the small gear very fast to give the large gear on the back wheel as much power as possible, but less speed.  Also, it feels easier to pedal.

In the simulation, you can feel different torques (rotational forces) between turning the large gear and turning the small gear ( 2 to 1 or 4 to 1 gears). In the example of the 2 to 1 gears, the small gear turns two times for a single turn of the large gear.  With the 4 to1 gears, the small gear turns four times for a single turn of the large gear.  You can feel the associated ratio of torques in each case by switching the handle from one gear to the other.