GO IN DEPTH

# Foucault's Pendulum

#### What You Need Ben Ostrowsky, Foucault Pendulum, CC-BY-2.0 via Wikimedia Commons

### Purpose

To understand how the motion of the Foucault pendulum proves that the earth is rotating.

### Context

This lesson will help to expand students' concepts of motion and gravitational force. Motion is as much a part of the physical world as matter and energy. Everything moves—atoms and molecules; the stars, planets, and moons; the earth and its surface and everything on its surface; all living things, and every part of living things. Nothing in the universe is at rest.

Most of the time, the relative motion of the earth does not affect our perception of motion. Since everything is moving, there is no fixed reference point against which the motion of things can be described. All motion is relative to whatever point or object we choose. Thus, a parked bus has no motion with reference to the earth's surface; but since the earth spins on its axis, the bus is moving about 1,000 miles per hour around the center of the earth.

There is no point in space that can serve as a reference for what is actually moving. However, in 1848, Jean Foucault discovered that when a large pendulum swings, the earth appears to "move under it." In this lesson, students will explore online resources to understand how Foucault used pendulums to show that the earth spins.

To prepare for this lesson, students should understand that everything in the universe exerts gravitational forces on everything else, although the effects are readily noticeable only when at least one very large mass is involved (such as a star or planet). Gravity is the force behind the fall of rain, the power of rivers, the pulse of tides; it pulls the matter of planets and stars toward their centers to form spheres, holds planets in orbit, and gathers cosmic dust together to form stars. Gravitational forces are thought of as involving a gravitational field that affects space around any mass. The strength of the field around an object is proportional to its mass and diminishes with distance from its center. For example, the earth's pull on an individual will depend on whether the person is, say, on the beach or far out in space.

Changes in motion—speeding up, slowing down, changing direction—are due to the effects of forces. Any object maintains a constant speed and direction of motion unless an unbalanced outside force acts on it.

Students should begin to understand how the principle of universal gravitation explains the architecture of the universe and much that happens on the earth. The principle will become familiar from many different examples (star formation, tides, comet orbits, etc.) and from the study of the history leading to this unification of earth and sky.

It is important for students to escape the common misconceptions that the earth's gravity does not extend beyond its atmosphere or that it is caused by the atmosphere. (Benchmarks for Science Literacy, p. 96.)

Students will better understand these concepts by exploring Foucault's experiments with pendulums.

Before you start this lesson with your students, you may want to review the information on About Foucault Pendulums, from the California Academy of Sciences, for yourself.

### Motivation

Before asking students to do an Internet exploration about pendulums, discuss with them their current knowledge, addressing any misconceptions. It is recommended that students already have had some instruction on Newton's law, which states that when a body is set in motion, it will move in a straight line from its origin, as long as it is not influenced by outside forces. This was the concept upon which Foucault based his proof that the earth rotates.

During the initial discussion, ask students:

• Why is it important to understand the effects of gravity and motion?
• (Gravitational force helps explain the architecture of the universe, such as star formation and comet orbits, and much that happens on the earth, such as tides, weather patterns, etc.)
• What ideas do you have about gravity?
• What common misconceptions did people use to have about the earth as it relates to gravity and motion?
• (Some misconceptions included ideas that the earth was flat, the earth did not rotate, heavier objects fell faster than lighter objects, objects in motion would eventually slow down and stop moving, etc.)

Now students should use their Foucault's Pendulum student esheet to view the Foucault's pendulum animation in the video at the top of the esheet.

• What observations can you make about the motion of the pendulum?
• (It appears to rotate.)
• How do pendulums help us understand gravitational force?
• (Pendulums help us understand the law of inertia and the rotating motion of the earth.)
• How does the Foucault pendulum demonstrate the rotation of the earth?
• (It demonstrates that the pendulum moves in the same path, but earth moves under the pendulum.)

Next, let students experiment with the Zoom Pendulum, on the PBS website. Although this experiment is intended for a younger audience, it serves as a good introduction to this lesson. In this experiment, students can change a pendulum's string length, the starting angle from which the bob is released, the rate of rotation, and the force of gravity.

• Define a pendulum.
• (Student responses may vary. Encourage students to explain their answers.)
• What makes a pendulum swing?
• (The force of gravity makes the pendulum swing.)
• What variables affect the pendulum's swing?
• (String length, angle, rate of rotation, and force of gravity affect the pendulum's swing.)
• How is the Zoom Pendulum different from a real pendulum that would be built on earth?
• (The Zoom Pendulum is not affected by the friction caused by the air on earth that would slow a pendulum down.)

### Development

In this part of the lesson, students should use their student esheet to explore the About Foucault Pendulums website, from the California Academy of Sciences, to learn how the scientist, Jean Foucault, was able to prove that the earth rotates by building and observing the motion of a pendulum.

Distribute the Foucault's Pendulum student sheet. Students should work together in pairs so they can help each other understand the facts and concepts in the online activity.

As students read each 1-2 page section, they should answer the questions on the worksheet. After each section, lead a discussion, answering the questions on the worksheet and helping students process the ideas. Make sure students understand each section before continuing.

Please see the Foucault's Pendulum: Answer Key for the questions from the student worksheet with suggested responses.

### Assessment

After students have explored the online resource or read the print article, students should be able to explain the following statement:

"In 1851, the French scientist Jean Foucault hung a large iron ball on a wire about 200 feet (60 meters) long. With this pendulum, he showed that the earth rotates on its axis. A Foucault pendulum is similar to a simple pendulum, but its motion is not limited to a plane. The plane of its swing appears to change as the earth goes through its daily rotation. However, the pendulum actually continues to move in the plane in which it was set in motion, while the earth turns under it."

Ask each student to write in his or her own words, an explanation of how a Foucault pendulum proves that the earth rotates.

### Extensions

Suggest to students that they visit a Foucault pendulum in person. There are over 60 Foucault pendulums installed around the world. A list of locations can be found at the California Academy of Sciences website.

The following Internet resource can be used to further explore topics related to pendulums, motion, and gravity:

• The Physics Classroom has a series of Internet lessons and animations on gravity and relative motion that will help to extend student understandings of these concepts. It includes lessons on such topics as Newton's Laws, Vectors—Motion and Forces in Two Dimensions, Momentum and Its Conservation, Work, Energy, Power, and others.  