Teacups to Space Stations: The Secret Science of Spinning

I’m Ken Kuwako, your Science Trainer. Every day is an experiment!

Have you ever been on a spinning ride at an amusement park, like the teacups, or a roundabout at a playground, and felt that weird sensation of being pulled strongly outwards? What exactly is the identity of that force that makes you want to shout, “Whoa, I’m gonna be thrown off!”?

The truth is, this force is a slightly mysterious one, known as a “fictitious force.” By reading this article, you’ll not only fully understand what it is but also see how it’s connected to amazing technologies that support our daily lives—from your washing machine to a future space station!

Now, let’s explore the world of rotation from a scientific perspective! First, experience this mysterious force for yourself in the video below.

The Secret to Circular Motion Seen from the Outside: “Centripetal Force”

First, let’s consider the teacup ride from the perspective of an outside observer. The children in the cup are moving in a circle (circular motion), aren’t they? In the world of physics, for an object to curve instead of moving straight, a force that continuously pulls it toward the center of the circle is always required. This force is called Centripetal Force.

In the case of the teacup, the wall of the cup is constantly pushing the child inward. This “pushing force” acts as the centripetal force, allowing the child to keep spinning in a circle without flying off. This is the same principle as the phenomenon where water doesn’t spill when you swing a bucket over your head in a circle. Your arm is constantly pulling the bucket toward the center (Centripetal Force), which keeps the water from spilling.

Expressed by the fundamental rule of physics, the equation of motion, this relationship is as follows:

ma = f (Change in the object’s state of motion = Force pulling toward the center)

A closer look at this equation reveals that the faster the spinning speed, the more the centripetal force required to maintain the circular motion increases by the square of the speed. This means if the speed doubles, the required force quadruples! This is why you have to hold on tight on high-speed spinning rides.

What Appears When Seen from the Inside: The True Nature of “Centrifugal Force”

Now, imagine you are riding inside the teacup. How does the scenery look? You are stationary in your seat, and the surrounding scenery seems to be spinning around you. You also feel your body being strongly pressed toward the outside of the cup, meaning away from the center of the circle. This mysterious force that feels like it’s pulling you outward is called Centrifugal Force.

図の引用：『これだけ!高校物理 力学編』（秀和システム）　桑子研

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Wait a minute, that force didn’t exist when we looked from the outside, did it? Actually, the true identity of centrifugal force is a type of “Inertial Force.” It’s the same kind of force that makes your body feel like it’s falling backward when a train accelerates suddenly, or falling forward when it brakes suddenly.

Objects have a property (Inertia) that makes them “want to continue their current state of motion.” While undergoing circular motion, you want to continue moving in a straight line, but you are being forced to curve inward by the centripetal force. As a result, you, the person inside the ride, feel the reaction to this as the “force pulling outward (Centrifugal Force).”

In other words, centrifugal force is a “ghostly, fictitious force” that is only felt by the person moving along with the ride.

From your perspective inside the ride, you are at rest. Therefore, you can think of the “force toward the center (Centripetal Force)” and the “force away from the center (Centrifugal Force)” as being in balance.

f (Centripetal Force) = ma (Centrifugal Force)

Fascinatingly, the mathematical equation looks the same whether you view it from the outside (Equation of Motion) or the inside (Balance of Forces). However, the meaning of that equation is completely different depending on which viewpoint you take. This is what makes physics so profound and interesting!

Centrifugal Force is Amazing! From Washing Machines to Space Stations

Although centrifugal force is a “fictitious force,” it is a very important force essential to our daily lives and future technology.

 
• Washing Machine Spin Cycle 🧺 The drum spins at high speed, generating a powerful centrifugal force. This presses the clothes against the drum, and the water is vigorously flung out through the holes in the side, resulting in dewatering (spinning).

 

• Salad Spinner 🥗 This uses the same principle as dewatering. The centrifugal force throws off the water clinging to the vegetables, giving you a crisp salad.

 

• Future Space Stations 🛰️ Living in zero gravity for long periods causes problems like bone weakening in the human body. Research is being conducted on the idea of rotating the entire colossal space station to use the resulting centrifugal force as “artificial gravity.” Amusement park spinning rides might just be a rehearsal for future space life!

Thus, centrifugal force appears simply by changing your perspective. The next time you get on a spinning ride at an amusement park or playground, be sure to feel this mysterious force in your body and let your mind wander to the vastness of space!

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