Recreate the Solar System with a Steel Ball and Magnet! Feel the Motion of Planets with a Conical Pendulum
I’m Ken Kuwako, your Science Trainer. Every day is a new adventure in discovery!
“Why do planets race around the Sun when they’re close, but drift more slowly when they’re far away?”
To bring this incredible mystery of space down to Earth, I built a simple hands-on experiment you can try in the classroom. All it takes is a steel ball, a piece of string, and a magnet. Hidden inside this tiny setup is the same universal rule Johannes Kepler uncovered over 400 years ago.
The Magnet Becomes the Sun, the Ball Becomes a Planet
This experiment is a planetary motion model based on a conical pendulum. A steel ball is tied to a string, while a magnet sits directly underneath. In this miniature solar system, the magnet plays the role of the Sun, and the steel ball acts as a planet.
As the ball swings in a circular path, something amazing happens: it speeds up as it passes close to the magnet and slows down as it moves farther away. Watching the motion with your own eyes makes the behavior of real planets suddenly feel easy to understand.
You’re Watching Kepler’s Law Come Alive
This motion perfectly mirrors the discovery made by the 17th-century astronomer Johannes Kepler. After studying enormous amounts of astronomical data, Kepler realized that planets move faster when they are closer to the Sun. This became known as Kepler’s Second Law, the Law of Equal Areas.
The name sounds complicated, but the core idea is surprisingly elegant. Imagine drawing a line from the Sun to a planet. Over equal intervals of time, that line sweeps out the same area every time. Near the Sun, the planet moves quickly through a short section of orbit. Far away, it travels more slowly across a longer path. Either way, the swept area stays constant.
In this experiment, the magnet imitates the pull of the Sun’s gravity, allowing the same orbital behavior to appear right in front of you.
The Secret Behind the Speed Changes: Energy
The real key to this phenomenon is energy conservation. As the steel ball approaches the magnet, magnetic potential energy transforms into kinetic energy, making the ball accelerate. As it moves away, the reverse happens: kinetic energy changes back into potential energy, and the motion slows down.
Real planets behave in exactly the same way. A planet’s motion is controlled by a constant exchange between gravitational potential energy and kinetic energy, while the total amount of energy never changes.
For billions of years, ever since the birth of the universe itself, planets have continued this endless cosmic energy dance.
A Classroom Experiment That Instantly Grabs Attention
Many teachers struggle to make planetary motion feel visual and intuitive. But this experiment can be assembled quickly using only a magnet, a steel ball, and some string.
The moment students see the model moving, their curiosity switches on. Suddenly, abstract textbook diagrams transform into something real and dynamic. One of the best things about this setup is how it makes the enormous scale of the universe feel surprisingly close and understandable.
Give it a try in your own classroom or at home!
Questions, Collaborations, and More
Making science fun, exciting, and approachable for everyone! I share easy science ideas, experiments, and teaching tips you can enjoy at home or in the classroom.
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５月のイチオシ実験！
キーンと冷えるドライアイス！気温が上がってくるこの時期・ドライアイスを使った昇華・凝結・等速度直線運動の実験はいかが？
液体ゼロ！ドライアイスが消えるまでの３時間を科学する（昇華・凝結・等速度直線運動）
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