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# Rotation

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Author: Janice VanCleave

### The Spinning of Celestial Bodies

Rotating celestial bodies spin around an axis, an imaginary line running through their centers. In 1851, French physicist Jean-Bernard-Léon Foucault (1819– 1868) interpreted the motion of a pendulum as proof that Earth rotates on its axis. A pin at the end of his pendulum made marks in sand on the floor of the Panthéon in Paris. As the minutes passed, the direction of the pendulum remained the same, but the marks underneath it changed. This proved that the floor moved beneath the pendulum as a result of Earth's rotation.

In this project, you will determine how a pendulum moves at the North and South Poles. You will determine if the length of a Foucault pendulum affects the results. You will also learn how the apparent shift in the path of a pendulum varies at different locations on Earth.

### Getting Started

Purpose: To model the motion of a pendulum at the North Pole.

### Materials

• marker
• sheet of white copy paper
• 10-inch (25-cm) string
• metal washer
• clothes hanger

### Procedure

1. Draw a large star on the paper.
2. Tape the paper to a wall.
3. Tie one end of the string to the washer.
4. Tie the free end of the string to the center of the hanger.
5. Holding the hanger so that one end of it faces the star, lift the washer in the direction opposite the star on the paper. Release the string and let it swing toward and away from the star (see Figure 7.1).
6. While the washer is swinging, slowly rotate the hanger about one-fourth of a turn in a counterclockwise direction. Observe the direction that the swinging string moves in relation to the hanger.
7. Rotate the hanger another one-fourth of a turn in a counterclockwise direction and again observe the direction that the swinging string moves in relation to the hanger.
8. Repeat step 7 two times, bringing the hanger back to its original position.

### Results

The string continues to swing toward and away from the star. In relation to the hanger, the pendulum appears to move in a clockwise direction.

### Why?

This investigation represents the motion of a pendulum (a weight that is suspended from a point and is free to swing back and forth) placed at the North Pole. The imaginary north-to-south line through the center of a body and about which it rotates is called an axis. The ends of the axis of a celestial body are called poles. The north and south end of an axis are called the North Pole and South Pole, respectively. The clothes hanger represents Earth rotating (turning on its axis). As viewed from above the North Pole, Earth rotates in a counterclockwise direction, like the turning hanger. When set in motion, the washer-andstring pendulum swings in one direction because of inertia (the tendency of an object at rest to remain at rest or of an object in motion to continue moving in a straight line unless acted on by an opposing outside force). In relation to the star, the swinging pendulum did not change direction, but in relation to the hanger, the pendulum seemed to move in a clockwise direction. This is because the hanger rotated in a counterclockwise direction. Thus, the pendulum's apparent motion was opposite to the rotation of the hanger.

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