Threads: How is a Screw Like an Inclined Plane?

Problem

How is a screw like an inclined plane?

Materials

• Pencil
• Sheet of typing paper
• Ruler
• Scissors
• Marking pen
• Cellophane tape

Procedure

1. Draw a right triangle on the sheet of paper with a base of 4 inches (10 cm) and a height of 6 inches (15 cm).
2. Cut out the triangle.
3. Color the diagonal edge of the paper triangle with the marker.
4. Tape the triangle to the pencil, with the colored edge facing up as shown in the diagram.
5. Rotate the pencil to wrap the paper tightly around the pencil.
6. Tape the end of the wrapped paper to itself.
7. Count the number of diagonal stripes made by the colored edge of the triangle that is wrapped around the pencil.

Results

There are four diagonal bands spiraling around the pencil.

Why?

A screw is an inclined plane (a sloping or slanting surface) that is wrapped around a cylinder to form spiraling ridges. Screws look like spiral staircases. A common example of a screw is a wood screw. As this screw rotates, it moves into the wood a certain distance. This distance depends on the screw's pitch (the distance between the ridges winding around the screw). Each colored band on the paper around the pencil represents a spiral ridge on a screw, which is called a thread. Screws with less distance between the threads are easier to turn.

Let's Explore

1. Would the slope of the paper triangle affect the number of threads? Repeat the experiment using a triangle with a base of 2 inches (5 cm). Repeat the experiment again, this time using a triangle with a base of 6 inches (15 cm). Science Fair Hint: Display the models formed by the different-sized triangles.
2. Does the size of the pencil affect the results? Repeat the original experiment, replacing the pencil with a thinner rod, such as a knitting needle. Repeat the experiment again, this time using a rod thicker than the pencil, such as a marking pen. Science Fair Hint: Display the models formed and/ or diagrams of the models.

Show Time!

1. To demonstrate how a nut moves on and off of a bolt, hold the bolt in one hand. Put two fingernails on a thread in the center of the bolt. First rotate the head of the bolt in a clockwise direction, and then rotate the head in a counterclockwise direction. Observe the movement of the bolt through your fingers.
2. What are some other common examples of screws? You can display diagrams showing uses of different types of screws, such as:
   • "C" clamps holding materials together.
   • wood screws in a door hinge.
   • a screw jack raising a car.
   • a cork being removed from a bottle with a corkscrew.

Check It Out!

A screw is a disguised form of an inclined plane. Find out more about the movement of different types of screws, and compare them to the movements of other similar inclined planes. How do screws, like all inclined planes, affect force and distance? Include a comparison between a wedge and a wood screw; these inclines both produce strong forces at right angles to their movement.

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