The Coriolis Effect

The Coriolis force comes from the rotation of Earth. Earth spins on its axis at a rate of one rotation per 24 hours. At the equator, this is equivalent to approximately 1,600 km per hour—this is the speed a person standing at the equator experiences. But at the North and South Poles, the speed is zero. This differential in speed causes eddies (swirling patterns) in the atmosphere. These in turn affect weather patterns.

Put a few drops of food coloring on a tennis ball, gently lower it into a tub of water, and give it a spin with your fingers. Note the patterns of motion that the food coloring makes in the water.

Hurricanes spin counterclockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere because of the Coriolis force.

We don't notice the spinning of Earth directly, because we move at constant velocity (speed and direction).

A popular myth holds that the water in toilets and sinks demonstrates the Coriolis effect (the observed effect of the Coriolis force) by draining counterclockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere. However, this actually has to do with the design of the toilet or sink rather than where it is located on Earth.

NASA scientists must take Coriolis effects into consideration when they launch rockets. In fact, space launching facilities, including the Johnson Space Center in Houston and Cape Canaveral in Florida, are located in the south to take advantage of the greater speed of Earth's surface at those latitudes (distances north or south of the equator, measured by imaginary lines running east to west parallel to the equator). This activity shows you several ways to demonstrate the Coriolis force and its effects.

Materials

• 2-L soda bottle
• water
• food coloring
• metric ruler
• merry-go-round or a swivel chair and weights
• safety goggles
• steel washer, nut, or other small weight
• I-m nylon fishing string or line

These effects were first described by GaspardGustave de Coriolis (1792–1843), a French engineer and mathematician.

Procedure

1. Fill a 2-L soda bottle with water. Turn it upside down and let the bottle begin to pour out. Swirl the bottle clockwise until a miniature cyclone starts. Study the cyclone as the water pours out. Notice that the swirl will remain powered by gravity even if you hold the bottle still. For a more dramatic effect, first release a drop of food coloring from a height of 10 cm and allow it to settle into the water. As an extension, you can vary bottle sizes and mouth openings to find out what conditions work best to support this motion.



2. Get on a small merry-go-round and give it a good spin. Move toward the center. Notice what happens to the rate of rotation. You spin faster because of a principle called the conservation of angular momentum.

Angular momentum is a quantity that is based upon an object's mass and rate of rotation.

Move back to the edge and the spinning slows down. You can demonstrate the same effect in a swivel chair by holding weights in your arms, spinning, and then moving your arms toward and away from your body, or by observing figure skaters as they change their rate of rotation using their arms.

3. Put on your safety goggles, and swing a small weight in a circular orbit at the end of a I-m string. Let the string wind around your finger as shown. The result is always the same—as the length of the string decreases, the speed of the weight increases. The string may be compared to a nearly massless merry-go-round and the weight to a heavy person.

In physics terms, the weight has a radial velocity (speed of rotation in respect to angle) toward your finger because of the shortening of the string. The radial velocity interacts with the rotational velocity (the speed and direction the weight turns) to produce an acceleration that is tangential (touching but not intersecting) to the path of the weight and acts to speed up the weight.I

As artificial satellites fall toward Earth out of their orbit, the radius of their orbit (the distance to Earth's center) decreases and their speed increases, until friction becomes so great that they burn up in the atmosphere.

References

Rocket Science: 50 Flying, Floating, Flipping, Spinning Gadgets Kids Create Themselves by Jim Wiese (New York: John Wiley & Sons, 1995).

The Spinning Blackboard and Other Dynamic Experiments on Force and Motion (Exploratorium Science Snackbook series) by Paul Doherty (New York: John Wiley & Sons, 1996).

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