Foucault's Pendulum


You are probably familiar with Newton's first law of motion—or his law of inertia. Did you know, though, that scientists used this law to help prove that the earth rotates? Use the resources on this student esheet to learn more about Jean Foucault and how he used a pendulum to help prove the earth rotates.


Vie the Foucault's pendulum animation at the top of this page. When you are done watching this animation, answer these questions on your Foucault's Pendulum student sheet:

  • What observations can you make about the motion of the pendulum?
  • How do pendulums help us understand gravitational force? 
  • How does the Foucault pendulum demonstrate the rotation of the earth?

Now go to Zoom Pendulum to experiment with the animated pendulum on that site. Once you're done experimenting with the pendulum, answer these questions on your student sheet:

  • Define a pendulum.
  • What makes a pendulum swing?
  • What variables affect the pendulum's swing?
  • How is the Zoom Pendulum different from a real pendulum that would be built on earth?

With your partner, visit About Foucault Pendulums to learn how the scientist, Jean Foucault, was able to prove that the earth rotates by building and observing the motion of a pendulum. 

As you go through each 1-2 page section, answer the corresponding questions on your student sheet:

What’s a pendulum anyway?

  • What is the definition of a pendulum?
  • What are the parts of a pendulum?
  • What effect does inertia have on a pendulum?
  • Why is a pendulum scientifically important?
  • Why can pendulums be used for time keeping?
  • Why is measuring the acceleration of gravity or “g” important?
  • How do you think a pendulum can be used to show that the earth spins?

Wrong ideas: what people used to believe about the earth

  • What were some of the early ideas about the earth that were later proven wrong?
  • Who were some of the first scientists to believe the earth rotates?

Early experiments to prove the rotation of the earth

  • What were some early experiments that try to prove the rotation of the earth?
  • Why were these experiments inconclusive?

Foucault’s three pendulum experiments

  • Describe Foucault’s three experiments. In each experiment, what were the length of the wire and the weight of the bob?
  • Why did the longer wire result in longer and slower oscillations?
  • What did Foucault use to show the how the plane of oscillation moved?
  • Explain how this proved that the earth rotates.

How the Foucault pendulum works (California Academy of Sciences pendulum)

  • Why would a pendulum normally stop after a few hours?
  • What keeps the pendulum moving?

Why does the pendulum demonstrate the rotation of the earth?

  • Describe how each of the following affects the movement of the pendulum:
    1. 1. Inertia
    2. 2. Gravity
    3. 3. Air Resistance
  •  Why does the pendulum demonstrate the rotation of the earth?

What are the different kinds of motion around the earth’s axis?

  • Describe the motion of a pendulum and the building around the earth’s axis under the following conditions:
    1. 1. Perpendicular axis at the North Pole
    2. 2. Parallel axis at the Equator
  •  What happens if the pendulum was at latitudes between the North Pole and the Equator?

How the rotation of the earth affects our lives

  • Describe how the earth’s rotation affects the following:
    1. 1. Plane flights
    2.  2. Weather

How to figure the period of a simple pendulum

  • Discuss the equation to calculate the period swing of a pendulum. What does each part of the equation represent?
    1. 1. What is the period of swing?
    2. 2. Define what is meant by the acceleration of gravity.

How to figure the number of degrees of rotation of the earth beneath the pendulum in 24 hours.

  • Discuss the equation to calculate the number of degrees the earth rotates beneath a pendulum in 24 hours. What does each part of the equation (n = 360 degrees x sin of latitude) represent?

This esheet is a part of the Foucault's Pendulum lesson.

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Esheet Details

Grades Project 2061 Benchmarks National Science Standards