Here's another example. Think about a wrench. The circle you make by pulling on the handle is the wheel. The axle is where the head of the wrench grabs a bolt. A smaller force applied to the handle can move a larger load attached to the axle.

The same kind of thing happens when you turn a doorknob. The knob is a wheel and is connected to a rod, which is the axle. When you turn the knob, the rod pulls on a latch that moves out of the doorframe. Now, you can open the door.

With simple machines, there are always trade‑offs. The larger the wheel, the less force you have to use. But you have to use that force over a greater distance. So, you trade distance for force. That’s not a bad trade!

Bikes have many wheels and axles— the wheels and pedals, for example. Bikes also have toothed wheels called gears. A chain connects a gear around the pedals with one or more gears on the back wheel.

When you pedal, the chain turns the rear axle. So, the rear wheel turns, too. The wheel is much larger than the axle. So, the wheel turns farther and faster than the axle. So does the front wheel. It sort of “goes along for the ride.” Wheels and axles make work easier—and more fun!