Coriolis Effect

Introduction   The Centrifugal "force", discussed in the previous page, is one manifestation of inertia that you feel when you are turning. There is another. It's called the Coriolis Force or, better, the Coriolis Effect. Both can be thought of as by-products of the fact that the reference frame (or coordinate system) is itself accelerating, because it is rotating. So, how does the Coriolis Effect show itself?

Imagine you are on a humongous turntable, like an old-fashioned record player built for giants. The table is turning counterclockwise, and you are sitting near the edge, facing inwards. You throw a tennis ball, aiming directly for the center of the turntable. The ball has two components of velocity, radial (towards the center) and tangential (sideways, perpendicular to the radius). The tangential velocity of the ball, as the ball travels towards the center, is greater than the tangential velocity of the turntable surface over which it is flying. So, to you, the ball will appear to curve rightwards and miss the center.

To someone standing on the ground, not spinning, the ball will appear to fly in a straight line. It misses the center because, although you tried to throw it towards the center, you forgot that the ball was already moving to the side.

In the northern hemisphere, hurricanes rotate counterclockwise. Why? First, remember that the Earth is spinning towards the East. Also, the part of the Earth's surface that is moving the fastest is the equator. Hurricanes are intense low-pressure systems. The low pressure in the center tries to draw air inwards. Air that is south of the center, which is moving eastward due to the spin of the planet, is also drawn northwards by the pressure difference. As it moves north, it is also moving east, but faster than the ground over which it is passing. The air turns to the right, eastward. Air that is north of the hurricane's center is also being drawn inwards, towards the south. But that air's eastward velocity is slower than the land over which it is moving. Although the air is moving towards the east, it will lag behind the land, so relative to the land the air is turning to the west, to the right. So, airmasses moving in the northern hemisphere consistently turn to the right.

The same type of reasoning explains why airmasses in the southern hemisphere consistently turn to the left, causing hurricanes to turn clockwise.

Likewise, the shells from long-range cannons are deflected to the right in the northern hemisphere, and to the left in the southern hemisphere.

This video will help explain. (If the inline image below doesn't work, click here.)

It is a common myth that the water in toilet bowls flushes counterclockwise in the northern hemisphere, and clockwise in the southern hemisphere, just like hurricanes. However, the speed of the water, and the distances over which it moves, are not sufficient for the Coriolis Effect to be significant. The water swirls in the direction the channels molded into the toilet direct it, which is counterclockwise for the toilets from most manufacturers, regardless of what hemisphere the toilet is in.

[Insert video from turntable here.]

Both the centrifugal force and the Coriolis force are called fictitious forces. They are by-products of the object's inertia. What's another way to judge that they are not really forces? There is no other object exerting these forces, and no Newton's 3rd Law "reaction".

Activities & Practice
to do as you read

1. The Coriolis Effect is named after Gaspard-Gustave Coriolis, a French scientist who wrote about it in 1835. Learn more about Coriolis here.