Please read last month’s article for the first three simple machines: The lever, the pulley, and the wedge.
4. The Inclined Plane
Basically a fancy name for a ramp, an inclined plane makes up-and-down motion easier by spreading it over a distance. The most obvious use of an inclined plane is for transportation by pedestrians (stairs are difficult for a mobility-impaired person or a wheelchair) and vehicles (stairs are basically impossible for a car). The slope, or pitch, of the plane determines the mechanical advantage. It takes only half as much force to push an object up a 30-degree plane as it does to lift it straight up, but you’ll need to move the object twice as far.
Another use of an inclined plane is to slow down something that’s falling. A roller coaster doesn’t drop straight to the ground (that would kill you); it rolls down a ramp, which also gives it forward momentum and enables it to climb the next hill smoothly. When Isaac Newton developed his laws of motion, he slid various objects down an inclined plane to see how quickly they accelerated. Freefall happened too fast for him to observe with his 17th-century equipment.
5. The Screw
A screw converts circular force to linear force. You twist your screwdriver, or wrench, or drill… and it moves your screw, or bolt, or drill bit in a completely different direction. Which direction? Screw threads usually follow the right hand rule: if you twist in the direction your fingers curl on your right hand, the screw moves in the direction your thumb points. (Righty tighty, lefty loosey.) If you ever see a screw with left-hand threads, there is a good reason for it. For example, you’ll find a left hand screw inside your toilet that keeps the flusher from falling off with repeated presses.
As with most of the simple machines, a screw trades force for distance. Twisting a screwdriver might be 100 times easier than pushing a screw straight into a piece of wood, but your hand will need to move 100 times as far as the screw actually goes. The total, force x distance, is always preserved no matter what machine you use – a concept known as conservation of energy.
6. The Wheel
A wheel (with an axle) converts sliding movement to rolling movement. Think of how hard it is to drag a file cabinet across the floor, and how much easier it is if that cabinet is on a dolly. Sliding usually creates lots of friction because there’s a huge amount of surface area in contact. A wheel reduces that contact zone to a tiny area.
The irony is that even though a wheel reduces friction, it depends on friction to work in the first place. On a frictionless surface, a wheel can’t roll forward; it just spins in place. You know this if you’ve ever tried to drive on a wet or icy road. The wheels spin without moving forward, and the car doesn’t roll – it slides.
Wheels are commonly found in more complex machines. A gear is a series of wedges spaced equally around the outside of a wheel. A crank is a lever that turns a wheel at the fulcrum. And technically, most pulleys are actually a combination of a pulley and a wheel.
Here’s a fun question to ponder: if your car is moving forward at 5 feet per second, how fast is your tire tread moving? Place your bets now, and we’ll answer this question next time on Monthly Mechanics.