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magine needing to transport building materials, such as heavy slabs of rock, from a rock quarry to the site of your new building, miles away. How would you do so without a truck with wheels or even a rolling cart? Somehow people did so without the help of a wheel and axle for thousands of years!

Although the wheel and axle is often associated with very early humans or "cavemen," it was invented relatively recently. It was such a complex and challenging invention that it came much later in our time here on Earth, during a period of human history called the Bronze Age.

No one knows for sure how the wheel and axle was invented, but we do know that it was invented during a time when humans had already began constructing sailboats and casting metal alloys like bronze. Historians believe the wheel and axle dates back over 5,500 years and was probably first developed in the Middle East or possibly further north, in Eastern Europe. It's considered one of the six simple machines, along with the lever, pulley, inclined plane, wedge, and screw.

Can you imagine a world without the wheel and axle? Many jobs would be much harder and transportation would certainly look much different than it does today. In many practical ways, the wheel and axle makes the world go round (no pun intended!).

Some might think it's strange to call a wheel and axle a machine. In scientific terms, though, a machine is simply something that makes a force bigger. In other words, when you apply a force to a machine, it increases the size of the force and applies that greater force somewhere else.

For example, a hammer is a machine. Think of the job it does pushing a nail into a piece of wood. You could try as hard as you could without a hammer, but you would not be able to push a nail into a piece of wood with your bare hands. With a hammer, however, you're able to tap the head of the nail until it's firmly embedded in the wood.

So how does a wheel and axle make a force bigger? Think about going grocery shopping. You fill your cart to the brim and wheel it to the checkout lane and ultimately out to your car. Now think about how much force you'd have to use to move that cart if it didn't have wheels.

Can you imagine dragging it along the ground through the store and out to your car? The wheels multiply the force you use to push the cart, reducing friction and adding leverage along the way, to allow you to move a heavy load quite easily.

The wheel and axle can also multiply speed and distance. For example, when you ride a bicycle, you apply force to the axle. The axle, in turn, transfers that force to the attached wheel, which is much larger and allows you to move farther and faster than you could by simply applying force to the axle alone.

Other examples of the wheel and axle include door knobs, screwdrivers, steering wheels, and even egg beaters. Scientists call the amount by which a wheel and axle multiplies a force its mechanical advantage. The mechanical advantage of a wheel and axle can be calculated by dividing the radius of the wheel by the radius of the axle.

For example, if the radius of a wheel is 24 inches and the radius of its axle is four inches, then its mechanical advantage is 24 divided by four, or six. A mechanical advantage of six means the wheel and axle multiplies a force six times or allows you to accomplish a task using one-sixth the force you'd otherwise need.

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