It’s All About Trade‑offs
Think back to the different kinds of screws you used to build the doghouse. Even though the screws were the same length, the threads were different. The screws with more threads were easier to turn, but you had to turn them more often.
That’s the thing about a simple machine: there are always trade‑offs. The more threads a screw has, the less force you have to apply. But, you have to apply that force over a greater distance. You might be thinking, “What distance? I don’t go anywhere when I turn a screwdriver.” Your body doesn’t, but your hand does.
Think about the circle your hand makes when you turn the screwdriver. A screw with 30 threads means 30 turns of the screwdriver. Stretch out those turns. Add them up. Now you can see that you apply the turning force over a distance of several feet.
Do you want to cut down on that distance? Okay, but be prepared to supply more force with each turn. It’s like moving a stack of books from one desk to another. You can move the stack by taking a couple books at a time. You use little force to pick up the books, and you have to make more trips. Or you can move the whole stack at once. You use a lot more force, and it’s only one trip. If the whole stack is too heavy or too dangerous to carry, then it’s worth the extra trips.
It’s the same thing with the screw with fewer threads. You have to put in a lot more effort to provide the force it takes to turn the screw. Maybe it’s more effort than you can muster. Plus, fewer threads provide less friction to hold the screw in place.
So, when it comes to trade‑offs with this simple machine, it’s usually better to trade distance for effort. And that’s not a bad trade!
A mason jar and its lid have threads that fit together.
Hold on Tight
Screws come in many different shapes and sizes. How do you take off the lid from a jar of tomato sauce or pickles? You unscrew it! That’s because a jar lid is a screw. Unscrew the lid and take a look at the rim of the jar. You can see and feel the threads that spiral around the rim. Now look inside the lid. It has threads, too. They fit between the threads of the rim. The threads match up to grip and form a tight seal on the jar.
Can you think of other examples of screws that grip? How about the caps of water bottles and milk containers? Look through a kitchen pantry or refrigerator. You might find a lot of examples of how screws seal foods in their containers. That prevents spills and keeps the foods fresh.
Screws keep more than lids tight. What is the stem of a lightbulb? Correct! It’s a screw. The threads of the stem turn between the threads cut into the socket. Friction between these threads keeps the bulb in place. A good thing, too! Lightbulbs in the ceiling hang upside down. If they just slipped into place instead being screwed into place, they might fall out onto your head!
The threads on a lightbulb stem screw into the socket of a light.
Even a drill bit is shaped like a screw! Can you see the thread?
Dig In
When you drive a screw into a piece of wood, the job is easier if a narrow hole is already there. That’s where a drill helps. And guess what? A drill bit is a screw.
A motor spins the drill bit quickly. The tip of the bit is pointed so that it easily digs through the wood. In a couple seconds, you have a narrow hole that you can make a little bigger when you drive in the screw.
Larger drills make larger holes in the ground. Some drills dig through soil and rock to make wells for water or oil. Other drills make holes to set fence posts, telephone poles, and light poles.