Question

What are the directions of the centrifugal and Coriolis forces on a person moving (a) south near the North Pole, (b) east on the equator, and (c) south across the equator?

Short answer

(a) Centrifugal outward, Coriolis west. (b) Centrifugal outward, Coriolis downward. (c) Centrifugal outward, Coriolis west.

Step-by-step solution

Determine Centrifugal Force Near the North Pole

At the North Pole, the centrifugal force acts radially outward from the Earth's axis of rotation. Regardless of the person's direction of movement, the centrifugal force stays pointed away from the axis of rotation.

Analyze Coriolis Force Moving South Near the North Pole

For a person moving south near the North Pole, the Coriolis force acts to the right of the person's motion in the Northern Hemisphere. Therefore, as they move south, the Coriolis force will deflect them towards the west.

Determine Centrifugal Force on the Equator

At the equator, the centrifugal force also acts radially outward from the Earth's axis of rotation. This direction is perpendicular to the Earth's surface and constant, unaffected by the direction of travel.

Analyze Coriolis Force Moving East on the Equator

For a person moving east on the equator, the Coriolis force acts perpendicular to their velocity and towards the South Pole. However, because they are right on the equator, the effect of the Coriolis force is primarily to push them slightly downwards (hardly perceptible).

Determine Centrifugal Force South Across the Equator

As one crosses from north to south across the equator, the centrifugal force continues to act perpendicularly outward from the axis, independent of the crossing back and forth across hemispheres.

Analyze Coriolis Force South Across the Equator

For a person moving south across the equator, initially in the Northern Hemisphere, the Coriolis force is to the west (right of their path). Upon crossing into the Southern Hemisphere, the direction reverses, deflecting to the left, which means it still moves them westward.

Key concepts

Coriolis Force

The Coriolis force is an apparent force experienced by objects moving in a rotating system, like the Earth. It arises because of the Earth's rotation and causes moving objects to be deflected off their straight path. Think of it like turning a merry-go-round: if you toss a ball from the edge towards the center, the ball seems to curve rather than taking a direct line. On Earth, this deflection happens because the planet rotates as objects move over its surface.

• In the Northern Hemisphere, moving objects are deflected to the right.
• In the Southern Hemisphere, the deflection is to the left.

The Coriolis force is crucial for understanding weather patterns like the swirling motions of cyclones. It's also why we have trade winds shifting directions depending on the hemisphere. While the force doesn't directly alter an object's speed, it changes its trajectory, which affects navigation and metrology.

Earth's Rotation

The Earth rotates from west to east, which means it spins around an imaginary line called the axis. This rotation takes approximately 24 hours and is responsible for the cycle of day and night. The centrifugal force experienced due to this rotation makes anything on the surface seem lighter than it actually is. Because the Earth is not stationary, the size and direction of forces such as gravity, centrifugal force, and the Coriolis force vary across different locations. At the poles, the effect of Earth's rotation is minimized, while at the equator, it's maximized. Thus, traveling across the Earth's surface involves experiencing different effects of rotational forces depending on latitude.

Northern Hemisphere

When considering the impacts of forces in the Northern Hemisphere, it's important to remember how both the Coriolis and centrifugal forces behave. Moving south near the North Pole, a person will experience the centrifugal force pulling them outward, away from the axis. The Coriolis force will always try to deflect them to the right. This rightward deflection applies universally here, affecting airplanes, ocean currents, and wind patterns. It's why hurricanes spin counterclockwise in the Northern Hemisphere. The combination of both Coriolis and centrifugal forces helps shape the unique geophysical and meteorological phenomena we observe here.

Southern Hemisphere

In the Southern Hemisphere, the rules flip a bit. As the Coriolis force deflects objects to the left, say like moving north or south, it influences patterns like weather systems spinning in opposite directions compared to the Northern Hemisphere. For someone crossing the equator heading south, the Coriolis effect alters from right to left, still nudging them westward. Meanwhile, the centrifugal force acts uniformly outward from the Earth's axis, like it does all over the Earth.
Understanding such directional forces is essential for navigation and managing large-scale processes like oceanic circulation or atmospheric dynamics. Just like in the Northern Hemisphere, these forces in the Southern Hemisphere contribute to the grand dance of winds and currents.