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Rotation: The Spinning of the Earth on Its Axis

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Author: Janice VanCleave

The Earth rotates around an imaginary axis. The Poles of the Earth are at the ends of the axis. The North Pole is at the north end, and the South Pole is at the south end.

In this project, you will model a Foucault pendulum to examine the relation between the Earth's rotation and the inertia of free-swinging objects. You will calculate the apparent shift of the path traced by pendulums at different latitudes. You will also model the deflection of fluids due to the Coriolis effect.

Getting Started

Purpose: To model a Foucault pendulum.

Materials

  • 8-inch (20-cm) piece of string
  • metal washer
  • pencil
  • 1-quart (1-liter) jar
  • lazy Susan turntable
  • masking tape

Procedure

Rotation The Spinning of the Earth on Its Axis

  1. Tie one end of the string to the washer.
  2. Tie the free end of the string to the center of the pencil.
  3. Center the pencil across the mouth of the jar so that the washer is suspended inside the jar.
  4. Set the jar on the turntable.
  5. Spin the turntable counterclockwise and adjust the position of the jar and/or pencil so that the string hangs straight down as the turntable spins.
  6. Stop the turntable and secure the pencil to the jar with tape.
  7. Tilt the jar to start the washer moving back and forth in one direction. Set the jar back in place on the turntable.
  8. While the washer is swinging, spin the turntable in a counterclockwise direction as before (see Figure 3.1).

Results

The washer continues to swing back and forth in the same direction though the jar is turning around. In relationship to the turntable, the swinging washer appears to move in a clockwise direction.

Why?

The turntable represents the Earth rotating, which means it is turning about its axis (imaginary line that passes through the North and South Poles). As viewed from a position above the North Pole, the Earth rotates in a counterclockwise direction, like the turntable. The washer is a pendulum (a suspended object that swings back and forth). A pendulum swings in one direction because of inertia (the tendency of an object to remain stationary or to continue moving in a straight line unless acted on by an outside force). The swinging washer did not change direction, but the movement of the turntable made the pendulum seem to move in a clockwise direction, opposite to the rotation of the turntable. A pendulum placed at the North Pole would seem to move in a clockwise direction as seen by an observer on Earth, like the washer in the jar, even though inertia in fact keeps the pendulum swinging in one direction.

A similar experiment was first performed in 1851 by a French physicist, Jean-Bernard-Léon Foucault (1819 –1868). Foucault used a 223-foot (67-m) wire to suspend a sphere weighing 62 pounds (28 kg) from the dome of the Panthéon, a public building in Paris. The pendulum was free to move in any direction, but once it was set in motion, inertia kept it swinging in the same direction. A pin at the end of the pendulum made marks in sand on the floor. As time passed, the direction of the marks changed. Because of inertia, the pendulum swung in the same direction. Since the sand rested on the floor of the building, and the building rested on the Earth, the pendulum showed that the Earth itself moves. Thus, Foucault's experiment showed the rotation of the Earth. A pendulum that shows the rotation of the Earth is called a Foucaultpendulum.

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