The Coriolis Effect: Definition & Influences

Instructor: Joseph Marra
In this lesson, we'll be looking at the Coriolis Effect: what it is and how it influences the way we perceive the movement of objects while here on Earth. When you're done, you'll be able to test your newfound knowledge with a quiz.


The Coriolis Effect, described as a force that's acting on every object on the Earth, is often given credit for making the water in your toilet turn a certain direction. In truth, it is a matter of the perspective of the observer, but also has a very real component in several areas of science that should be understood. The Coriolis Effect helps describe the path we see certain objects take. To really get into this topic, we'll need to spend some time on the rules that govern the motion of objects.

Newton's Laws of Motion

Let's start with a very quick review of Newton's Laws.

Newton's First Law, often referred to as the Law of Inertia, says that an object will remain at rest or move with a constant velocity unless it's acted upon by an external force. Another way to say this is that, unless a force is acts on object, its speed and direction won't change.

Newton's Second Law of Motion flows from Newton's First Law of Motion and tells us that if a force does act upon a body, it'll change its velocity and direction. It also tells us that the change will be proportional to the force that's applied and inversely proportional to the mass of the object.

Now, there's a caveat to Newton's Laws: they only apply in what we call an inertial reference frame. This may seem a little abstract, but if you drove in your car today, you experienced this concept. And probably without even realizing it!

Let's use the inside of your car as our reference frame and we'll work to understand whether it's an inertial or non-inertial reference frame. Imagine you're driving to work and you leave your backpack on the front passenger seat. You're in a bit of a rush, so you're driving fast so you don't get caught at the next traffic light. Sure enough, it turns red just before you get there, so you slam on the brakes and the car comes to a stop. And of course, your backpack goes flying towards the front of your car.

Let's focus on the movement of that backpack, and we'll do it from the point of view of two observers.

First, let's imagine that there's a passenger in the car, and for whatever reason, that passenger doesn't ever realize the car is moving. As far as he's concerned, he's just sitting still in this box and all of a sudden, the backpack goes flying forward. Almost like a ghost just picked it up and threw it! This absolutely violates Newton's First and Second Laws, so how could this be?

Let's then imagine a person who's waiting at the stop light to cross the street. That person sees the car coming, and when you slam on the brakes, they see the backpack move towards the front of the car. From their perspective, it all makes sense because it obeys Newton's First and Second Law.

To the observer inside the car, the car is a non-inertial frame of reference. Because the frame of reference is moving, movement inside reference frame may not obey Newton's Laws. To the observer outside of the car, the car is an inertial frame of reference, as all the motion can be precisely described by Newton's Laws.

Fictitious Forces

Now that we've established what inertial and non-inertial reference frames are, we can start to understand what the Coriolis Effect is and why we have it.

Let's think back to driving in your car as a very simple case. Even though the backpack sitting on the front seat didn't obey Newton's Laws, we still want to understand why it's behaving the way it is. For example, if the backpack was a person, we'd want to understand what force that person was subjected to so we could design a proper safety belt.

This leads to a discussion of fictitious forces, one of which is the Coriolis Effect. Fictitious forces are ways that we describe the motion of objects in non-inertial reference frames by understanding why they seem to behave out of the ordinary to us as observers.

The Coriolis Effect

The Coriolis Effect is a fictitious force that enables us to correctly describe the motion of an object relative to a rotating reference frame. In most cases, we refer to the Coriolis Effect in the context of the motion of objects relative to the rotation of the Earth.

The path of an airplane flying from the North Pole to the Equator is the most classic example. As shown in the image below, imagine you're planning on taking a flight from the North Pole to the South Pole.

Coriolis Effect showing travel from North Pole to equator 1

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