We move the world
A propeller is a type of fan that transmits power by converting rotational motion into thrust. A pressure difference is produced between the forward and rear surfaces of the airfoil-shaped blade, and a fluid (such as air or water) is accelerated behind the blade.
If you want to move forward, you need to push backward; that fundamental law of physics was first described in the 18th century by Sir Isaac Newton and still holds true today. Newton's third law of motion (sometimes called "action and reaction") is not always obvious, but it's the essence of anything that moves us through the world. When you're walking down the street, your feet push back against the sidewalk to move you forward. In a car, it's the wheels that do something similar as their tires kick back against the road. But what about ships and planes powered by propellers? They too use Newton's third law, because a propeller pulls or pushes you forward by hurling a mass of air or water behind you. How exactly does it work? Why is it such a funny shape? Let's take a closer look!
Propellers, often shortened to "props," are sometimes called screws—and it's easy to see why. To push a screw into the wall, you apply a clockwise turning force to the head with your screwdriver. The spiral groove (sometimes called a helical thread) on the screw's surface converts the turning force into a pushing force that drives the screw into the wall and holds it there. But suppose, for a moment, that you wanted to keep on going...
If you were a beetle and you wanted to move through an infinitely long wooden wall, you could use a screw thread on the outside of your body to do it. You wouldn't need a screw running along the whole length of your body: you could manage with just a little thread on your head—a kind of screw cap—to bite into the wood in front of you. Now suppose you were a fly, not a beetle, and you wanted to go through air rather than wood. There's no reason why you couldn't use a screw thread in exactly the same way to pull you through the sky. In effect, you'd be a fly with a propeller—and that's pretty much what the first airplanes were. Planes took to the sky when the Wright brothers figured out how to combine engine-powered propellers and wings so they could go forward and upward at the same time.
A propeller is a machine that moves you forward through a fluid (a liquid or gas) when you turn it. Though it works the same way as a screw, it looks a bit different: usually it has two, three, or four twisted blades (sometimes more) poking out at angles from a central hub spun around by an engine or motor. The twists and the angles are really important.
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