It’s All About Trade‑offs
Remember building Ollie’s doghouse? The screws were the same length, but 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 simple machines. There are always trade‑offs. The more threads a screw has, the less force you use. But, you use that force over a greater distance. “What distance?” you ask. Your body doesn’t move, 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. You’ve applied the turning force over a distance of several feet.
Do you want to cut down on that distance? Okay, but be prepared. You will need to use 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, but you have to make more trips. Or you can move the whole stack at once. You use a lot more force, but it’s only one trip.
It’s the same thing with the screw. A screw with fewer threads has drawbacks. It requires more effort. Plus, it provides less friction to hold the screw in place.
A 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 pickles? You unscrew it! A jar lid is a screw. Unscrew the lid and take a look at the rim of the jar. You can feel the threads that spiral around. Now look inside the lid. It has threads, too. They fit between the threads of the rim. The threads match up and grip to form a tight seal on the jar.
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.
A drill bit is shaped like a screw! Can you see the thread?
When you drive a screw into wood, it’s 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 into the wood. In a few seconds, you have a narrow hole. You can make it bigger when you drive in the screw.
Some larger drills make large holes in the ground. The drills dig through soil and rock. They make wells for water or oil. Some smaller drills make holes to set fence posts and telephone poles.