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# Rotation: The Spinning of the Earth on Its Axis (page 2)

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Source:
Author: Janice VanCleave

### Try New Approaches

As seen from a position above the South Pole, the rotation of the Earth is clockwise. How would the apparent path of a pendulum be affected if the pendulum were placed at the South Pole? Repeat the experiment, rotating the turntable in a clockwise direction.

Make a model to demonstrate the curved paths of fluids due to the Coriolis effect. Cut an 8-inch (20-cm) circle from a sheet of paper. Use a paper punch to make a hole in the center of the paper circle. Insert a paper brad through the hole in the center of a ruler and the hole in the paper circle. Lay the paper circle on atable with the ruler on top. Use masking tape to tape the ends of the ruler to the table. Position the point of a ballpoint pen at the top of the paper and to one side of the ruler, as shown in Figure 3.2. Draw a line on the paper along the edge of the ruler with a black pen. This straight line shows the path of fluids without the Coriolis effect. Repeat the procedure using the same paper but a red pen. Align the black mark next to the ruler. With your red pen at the top of the paper as before, ask a helper to slowly rotate the paper circle counterclockwise as you draw a line on the paper as before. The curved red line shows the path of fluids with the Coriolis effect. Science Fair Hint: Use the drawings as part of a project display representing the path of fluids with and without the Coriolis effect.

1. To find sin(latitude), enter the latitude on a scientific calculator and hit the sine function key.

Example:

Foucault's original pendulum was located in Paris, which is at latitude 47° N. Calculate the apparent shift of the pendulum's path in degrees per hour.

Use the example to determine the shift of a Foucault pendulum at your latitude.

1. The path traced by a Foucault pendulum appears to shift as the Earth rotates. The amount of the apparent shift of the pendulum's path depends on the latitude of the pendulum's location. At the Poles (latitude 90°), the apparent shift of the pendulum's path is 15° per hour, but at lower latitudes the shift decreases and at the equator (latitude 0°), the apparent shift is 0° per hour—the path doesn't shift at all. The apparent shift (d) in degrees per hour can be calculated for a given latitude using the following equation:
• d = 15° sin(latitude)
• d = 15° sin(47)
•    = 15°(0.7314)
•    = 10.97° per hour
2. It takes 24 hours for the pendulum to make an apparent 360° shift at either the North or South Pole, but it takes longer at the lower latitudes. The time (t) it takes for the pendulum to make an apparent shift through a complete revolution of 360° at a specific latitude can be calculated by using this equation:

• t = 360°ld

where d is the apparent shift of the pendulum's path in degrees per hour, as calculated in the previous experiment.

Example:

Calculate the time for the path of Foucault's pendulum in Paris to shift 360°.

• t = 360°ld
•    = 360°/10.97° per hour
•    = 32.82 hours

Use the equation to determine the time it takes for a pendulum's path to apparently shift 360° at your latitude.

2. The Earth's rotation affects fluids (gases or liquids) such as winds and ocean currents that move freely across the Earth by causing them to deflect (turn aside from a straight path) and move in curved paths. This deflection of fluids as a result of the Earth's rotation is called the Coriolis effect.

### Get the Facts

The major wind patterns on the Earth are called prevailing winds. How does the Earth's rotation affect the direction of these winds? For information, see Janice VanCleave's Oceans for Every Kid (New York: Wiley, 1996), pp. 65–75.

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