Everybody enjoys a Screw! The Screw is simply an Inclined Plane wrapped around a cylinder, and it converts rotary motion into forward motion. Instead of pushing something up an Incline Plane, the screw allows you to push the Inclined Plane into the something -- imagine each turn of your screwdriver as a push on that box. Thinking up a nail was no great feat, if you ask me; but the brainiac who came up with the screw was a friggin' genius.
The Wheel turns me on! And speaking of things I'm glad someone invented ... The Wheel and Axle is, in essence, a rolling Inclined Plane. And why is it useful? Well, you'll recall that (a) the more surface contact two objects have the more friction you'll encounter when you try and move one, and (b) a circle only touches a tangential line at a single point. So moving an automobile forward with only four points touching the pavement (i.e., the four spots where the rubber hits the road) is a helluva lot easier that trying to move the thing forward with its entire underbelly scraping along the pavement.
Update! Reader Henry Stafford calls me to the carpet: "Ummm...you should do some googling on surface friction. Surface area has zero effect on the friction between two objects. For example, take a deck of cards lay it flat on the table, and push it. Now stand it up on one side (the deck of cards should still be in the box - did you just make a huge mess?) and try to push it across the desk. If you have properly calibrated finger-pushing-force sensors, you'll find you need the exact same amount of force to push the boxed deck of cards, whether it's on edge, or laying flat. A wheel is great because it isn't sliding at all, not because it's surface area is small." I strongly suspect that, unlike myself, Mr. Stafford actually knows what he's talking about. So listen to him, okay?
Wheels can also be given teeth and function as Levers -- that, my friend, is what we in the weblog business call a "gear". And what, pray tell, is this this mysterious thing called a "Lever"?
We're all pulling for you, Lever! The lever kinda does the same thing as an Inclined Plane: converts force over distance into increased quantity of force. Or it just changes the direction of force. It all hinges on the fulcrum, which is the point at which the lever pivots. Take a seesaw. Here we have a lever with a fulcrum at the exact center, so the machine just changes the direction of force (one kid goes down pushing the other kid up). That's your first-class lever right there. A second-class lever is one with an off-center fulcrum (such as a crowbar), allowing you to move the end farthest from the pivot point a greater distance to move the side closest to the fulcrum with greater force. The closer the fulcrum is to the end of the lever, the greater the multiplier of force. So with, say, a bottle opener -- where the fulcrum is just millimeters away from the end -- you can push your end down a long way and pop that bottle cap right off. Without levers we couldn't open microbrews, leaving us to consume naught but canned beer and Budweiser. And that's why the Lever is one of the most important tools in the Simple Machine repertoire.
I can't think of a good pun for the Wedge! The wedge converts downward force into lateral force: that is, when you strike the top of a wedge, the force you apply is redirected so that it is perpendicular to the blade. If you hit a log with an axe (which is essentially just a wedge on a stick), the downward force of your swing is instantly converted into outward force radiating from the blade, thereby splitting the wood in two. Or when someone pulls up on your underwear with great force, that effort is converted to lateral motion, pushing your buttocks outwards. Okay, this paragraph is bringing back a lot of repressed memories so I'm going to quit while I'm ahead.
Bully for the Pulley! Hey, here's another use for the Wheel. A pulley changes the direction of force -- you pull down on the lanyard and the flag moves up the flagpole. If you connect a series of pulleys, you can lift a heavy object using less force -- the trade-off, as always, is that you must apply your lesser force over a longer distance. It's pretty sure I just misused the word "lanyard," there, but it's okay -- no one read down this far! Ready for some slightly less simple machines -- like, you know, the Space Shuttle? head on over to How Stuff Works.Posted on September 02, 2002 to Research Day