Screws

Core Idea

A screw is a simple machine that is really an inclined plane wrapped around a rod. The spiral ridges on a screw are called threads. As you turn a screw, the threads pull it into the material, converting a turning motion into a straight push. Screws are great for holding things together tightly and for lifting heavy loads slowly, like in a car jack.

How It's Best Learned

Wrap a triangle-shaped piece of paper around a pencil to show how an inclined plane becomes a screw thread. Let students twist real screws into soft wood or foam and compare the effort to pounding a nail. Examine a jar lid to see how the threads work.

Common Misconceptions

• A screw is completely different from a ramp. (A screw is an inclined plane wrapped in a spiral around a rod.)
• Screws and nails do the same job the same way. (Nails are wedges pushed straight in; screws use threads to pull themselves in and hold much more tightly.)
• The only screws are the kind in a toolbox. (Jar lids, light bulbs, spiral staircases, and car jacks all use the screw principle.)
• Turning a screw takes less total effort than pushing a nail. (You use less force, but you turn over a longer distance — the total work is similar.)

Explainer

Take a close look at a screw and you will see tiny ridges spiraling all the way up to the point. Those ridges are called threads, and they are the secret to how a screw works. A screw is actually an inclined plane — a ramp — that has been wrapped around a rod in a spiral. You can prove this by cutting a right triangle from paper and wrapping it around a pencil. The slanted edge of the triangle spirals around the pencil just like a screw thread.

When you turn a screw, the threads bite into the material and pull the screw forward in a straight line. Each full turn moves the screw in by a small amount equal to the distance between threads. This means you are trading a lot of turning motion for a small but powerful push. That is why screws hold things together so tightly — once they are in, the threads grip the material and resist being pulled back out.

Nails and screws might look like they do the same job, but they work in very different ways. A nail is a wedge — you hammer it straight in and it pushes the wood apart. A screw uses its spiral threads to pull itself in gradually. Because the threads grip so well, screws are much harder to pull out than nails. This is why builders use screws for projects where parts need to stay put, like decks and furniture.

The screw principle appears in many places beyond the toolbox. When you twist a lid onto a jar, the threads on the lid match the threads on the jar, pulling the lid tightly closed. A spiral staircase is a giant inclined plane wrapped around a central pole — it lets you climb to a great height with a gentle slope. A car jack uses a screw to lift a car slowly but with enormous force. Anywhere you need to change a turning motion into a powerful straight push, you will find the screw at work.