Hold it in place or move it along. Screws do both jobs all day long.

By Glen Phelan

Simple
Machines

Explorer magazine’s six‑part series

Before you read

Simple Machines

Explorer magazine’s six‑part series

This edition

Human Journey

Physical Science

Before you do the science activity at the end of this article, think about how you will record the results.

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Problem

You and a friend are helping to build a house for your dog, Ollie. The wooden boards are cut. The holes are drilled. All you have to do is connect the boards to the frame. You grab a screwdriver and some screws and go to work.

The first turn of the screw is easy enough. But then it gets harder and harder to turn. You hold the screwdriver tighter and strain to turn the screw. Uhhhh! Grrrr! After a minute of struggling, the screw is still sticking halfway out. Ollie tilts his head. He must be wondering if his doghouse will ever get finished.

Solution

Meanwhile your friend is having a much easier time. She is already putting in her third screw. “Hey, why are your screws so easy to turn?”you ask with a touch of jealousy. She beams a big smile. “I guess I’m just stronger than you.”

“No, really. Our screwdrivers are the same size. The holes are the same size. Even the screws are the same size. So what’s the difference?”

Then you notice it. “Hey, look at this. The threads are different.”

The ridges, or threads, of your screw are much farther apart than on the screws your friend is using. Your friend’s screws have a lot more threads, and they are really close together. That must be it!

You try one of the screws. You have to turn it a lot more times, but they are easy turns. The screw goes in fast and holds the board tight to the frame. “That’s more like it!” you exclaim. “Now that I have the right tools, we’ll have this thing built in no time.”

“And I think Ollie is going to love his new house,” your friend adds.

Woof! Woof! Ollie thinks so, too.

It's Simple

What made the screws such good tools to hold the wood together? It’s simple! A screw is a simple machine. Like all machines, simple machines help us do jobs, usually by moving things faster, farther, or more easily than we can without the machine.

Simple machines have only a few parts. In fact, some are made of only one part. Yet, nearly every machine contains one or more of simple machines.

Think about a pair of garden shears. The sharp edges of its two blades are wedges. The blades are also levers. So is the handle. The pivot point on which the blades move is a screw. Other screws hold the parts together.

Simple machines can be combined in endless ways to accomplish tasks, from sharpening a pencil to rocketing into space. But each simple machine also helps us do work all on its own.

A Closer Look

A screw with wider spaces between threads will require more force but take less time to move.

less force, more time

more force, less time

threads

When you turn the screwdriver, the screw redirects the force forward so it goes into the wood.

The Turn of the Screw

A screw is a neat little machine. Those ridges, or threads, that curl around are all connected. They form a single ramp that wraps around a rod. It’s like a tiny spiral slide. That spiral is what makes a screw do its job.

Think about what happens when you twist a screw into a piece of wood. You place the tip of a screwdriver into the slit in the top of the screw. You turn the handle. That takes some effort. This effort provides the force needed to drive the screw into the wood.

When you turn the screwdriver, you direct the force in a circle. The screwdriver transfers this force to the screw, which also moves in a circle. But then something very interesting happens. The screw changes the direction of the force! As the screw turns, it changes force that goes round and round into force that goes straight forward.

That’s the whole point of a screw—it redirects force. With each turn of the screwdriver, another thread moves forward and cuts into the wood. And once the screw is in tight, it stays that way. Friction between the threads and the wood hold the screw with a fierce grip that’s hard to loosen. So, can you remove a screw? Sure, but don’t try to pry it out with the claw of a hammer.

The way to remove a screw is to simply turn the screwdriver in the opposite direction. The screw changes the circular direction of the force. Except this time, the screw redirects the force straight backward instead of forward.

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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.

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