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Coriolis effect

What’s happening here?

Relative to the ground, the ball is traveling in a straight line at constant speed.

So why does look like it is curving?


Once air has been set in motion, you’d imagine that this air would continue moving in a straight line, right?
But the Earth isn’t flat – it’s curved!
Okay, so air would move in a line “as straight as possible”, following the curve of the Earth’s surface.

That is called a great circle:

shortest path, between two points, along the surface of a sphere.

Yet when we watch a mass of air move, it always gets deflected, as if by some invisible force?!

This invisible (fictitious) force is called the Coriolis force, after the French scientist Gaspard-Gustave Coriolis (1835) who first figured out what was really going on.

Yes, the air masses do follow a great circle – but the land underneath them is rotating away – because the entire planet is rotating.

So even though the air is going “as straight as possible”, us folks stuck to the Earth (everyone who isn’t on an overhead space station) see the air as if it is deflected away.

In the inertial frame of reference (upper part of the picture), the black ball moves in a straight line. However, the observer (red dot) who is standing in the rotating/non-inertial frame of reference (lower part of the picture) sees the object as following a curved path due to the Coriolis and centrifugal forces present in this frame.

– Wikipedia

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A hockey puck is launched from the north pole. As it heads south, the earth turns to the east, causing the puck to appear to deflect to the west as viewed by an earthbound observer.
Blue: Inertial Great Circle
Red: Path on rotating earth
Gray: Path on stationary earth

– – – –


A hockey puck is launched from London toward the west, on a stationary earth.
The natural great circle motion of the puck takes it toward the equator, not along the original line of latitude, which we might normally call west.
The great circle path also coincides with the line of sight toward the west (projected radially down to the earth’surface).
Thus we must conclude that Costa Rica is due west of London.
Blue: Inertial Great Circle

– – –

A hockey puck is launched from Vancouver toward the east.
The inertial great circle path of the puck takes it south of the great circle path that the puck would follow on a stationary earth.
The earthbound observer attributes this deflection to the centrifugal and Coriolis forces.
Note that even the stationary earth path takes the puck south of the original line of latitude.
Blue: Inertial Great Circle
Red: Path on rotating earth
Gray: Path on stationary earth

Cyclone over India

The direction of the Coriolis effect differs in the northern and southern hemispheres


Does water go down the drain in the opposite direction, on the other side of the equator?

If the water was in a HUGE basin – miles across – and drained slowly? Then yes. But for most realistic cases?

No – the Coriolis effect is only large enough to move water over large scales (bodies of water miles across)

Toilets and sinks are so tiny, that the Coriolis effect is swamped out by random motions in the water.



Video demonstration: MIT Department of Physics, technical services group

Two demonstrators sit at either end of a rotating platform and toss a ball back and forth. When viewed from the rest frame (when the camera is mounted to the ground), the ball follow a straight line but doesn’t reach its target because during the ball’s flight the target rotates away. When viewed from the rotating frame (when the camera is mounted to the rotating platform), the ball appears to experience a force that pulls it away from the target.

This curved trajectory in the rotating frame is known as the “Coriolis Effect”, sometimes called the “Coriolis Force”, though it disappears in the rest frame. The Coriolis Effect can be seen in many situations where rotating frames are encountered, especially meteorology and astronomy. Atmospheric systems, for example, often follow circular patterns due to the Coriolis effect. Airplanes and missiles appear to follow curved trajectories when seen by observers on Earth as the planet rotates underneath.

Learning Standards

Next Generation Science Standards

MS-ESS2-6. Develop and use a model to describe how unequal heating and rotation of the Earth cause patterns of atmospheric and oceanic circulation that determine regional climates. [Emphasis is on how patterns vary by latitude, altitude, and geographic land distribution. Emphasis of atmospheric circulation is on the sunlight-driven latitudinal banding, the Coriolis effect, and resulting prevailing winds; emphasis of ocean circulation is on the transfer of heat by the global ocean convection cycle, which is constrained by the Coriolis effect and the outlines of continents. Examples of models can be diagrams, maps and globes, or digital representations.]

SAT Subject Test in Physics
Circular motion, such as uniform circular motion and centripetal force

2016 Massachusetts Science and Technology/Engineering Curriculum Framework
HS-PS2-1. Analyze data to support the claim that Newton’s second law of motion is a
mathematical model describing change in motion (the acceleration) of objects when
acted on by a net force.

HS-PS2-10(MA). Use free-body force diagrams, algebraic expressions, and Newton’s laws of motion to predict changes to velocity and acceleration for an object moving in one dimension in various situations

Massachusetts Science and Technology/Engineering Curriculum Framework (2006)

1. Motion and Forces. Central Concept: Newton’s laws of motion and gravitation describe and predict the motion of most objects.
1.8 Describe conceptually the forces involved in circular motion.

Earth and Space Science, High School
1. Matter and Energy in the Earth System
1.4 Provide examples of how the unequal heating of Earth and the Coriolis effect influence global circulation patterns, and show how they impact Massachusetts weather and climate (e.g., global winds, convection cells, land/sea breezes, mountain/valley breezes).

New York Physical Setting/Earth Science Core Curriculum
Standard 4: Students will understand and apply scientific concepts, principles, and theories pertaining to the physical setting and living environment and recognize the historical development of ideas in science.
Key Idea 1: The Earth and celestial phenomena can be described by principles of relative motion and perspective
1.1e The Foucault pendulum and the Coriolis effect provide evidence of Earth’s rotation.

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