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.