Now You See It, Now You Don’t! Feel Newton’s First Law with the Disappearing Coin Trick!
I’m Ken Kuwako, your Science Trainer. Every Day is an Experiment.
What image comes to mind when you hear the phrase, “The Law of Inertia”? Many people would probably answer, “That feeling when a bus slams on the brakes and you lurch forward!” The textbook definition is often difficult to grasp, stating that “an object will maintain its state of motion unless acted upon by an external force.” It’s tough even for adults to understand this from text alone. So, this time, I’m introducing a magic-like experiment that lets you instantly feel this often-confusing law of physics. It’s called the Daruma Otoshi-Style Inertia Experiment. Despite its simple setup, it’s an absolute showstopper—the kind of experiment that earns an eruption of applause in the classroom when you nail it. I hope you try it and experience the excitement!
Why is this Experiment So Interesting?
When I first introduce this experiment in class, I start with the textbook explanation. But the students’ eyes are still dull. That’s when I pull out the equipment for this demo. I myself failed several times when trying to record a video of it. The cause was the thickness of the ruler I was using. A standard wooden ruler was too thick, and the moment I struck it, the force was transferred not only to the coin but also to the jar on top, causing everything to collapse. However, when I later switched to a thin metal ruler, the world changed. The coin shot out cleanly, and the jar just dropped straight down and rested perfectly in its place. That feeling of success is exceptional! In a real class, one student I had perform the experiment succeeded on the very first try! Apparently, he had practiced repeatedly at home, and he was nicknamed the “Master of Inertia” in class.
Experiment Preparation
The key to success lies in choosing the right tools and having enough weight. Gather the following: A glass jar (A plastic bottle is possible, but glass has more mass (weight), which means the “force to stay put” is stronger, making success easier.) Water (Pour this into the jar to add even more weight. This utilizes the property of inertia that “heavier objects are harder to move”.) Toy coins (Available at stores like Daiso. Coins with a smooth surface work best.) A thin, metal ruler (It should have good repulsion and be resistant to bending when struck. This is the most crucial point!)
How to Perform the Experiment
Once prepared, it’s time for the main event. Stack 2-3 coins and place them on the desk so they are stable. Gently place the water-filled jar on top of the stack (Balance it carefully so the jar doesn’t fall).
Insert the ruler under the coins, and with no hesitation, quickly and horizontally strike them out!
did it go? If successful, only the coin will shoot out with force, and the jar will land right in its spot as if nothing happened. This is proof that the Law of Inertia (Static Inertia) is at work. Because the jar was at rest, it maintained the property of “wanting to stay put!” even when the coin underneath it instantly disappeared. As a result, it simply falls straight down due to gravity and does not move horizontally.
Bonus: A World Where Moving Objects Keep Moving
So far, we’ve discussed how “objects at rest stay at rest.” But what happens to an object once it starts moving? Using the same concept of “inertia,” let’s look at the Air Hockey Experiment. That’s the one you see in arcades. On a slippery surface, once an object starts moving, you can observe it gliding continuously even without applying further force.
you look closely, the air hockey table has countless tiny holes from which air is blown up. This air causes the puck to float slightly, which drastically reduces the friction with the table surface. To get a bit technical, the formula for kinetic friction is $F = \mu’N$ (coefficient of friction × normal force). What’s important here is that friction is determined not just by the object’s weight (Mg), but by the force pushing down on the surface (Normal Force N).By lifting the puck, the air makes the force pushing down on the surface nearly zero. As a result, the friction essentially vanishes. In a world without friction, an object will move forever. This is called “uniform linear motion.” On Earth, we are inevitably subject to friction and air resistance, but a place without their influence would be… space, of course! Experiments in space are a parade of the Law of Inertia. We also recommend this video by astronaut Mohri. Static inertia, like in the Daruma Otoshi game, and moving inertia. Connecting these two concepts helps us see the mysteries of the universe.How about the tablecloth trick next?
Contact and Business Inquiries
Let’s bring the wonder and fun of science closer to you! We’ve compiled easy-to-understand explanations of fun science experiments you can do at home, along with tips and tricks. Feel free to search around!・About the administrator, Ken Kuwako: Click Here・For various requests (writing, lectures, science classes, TV supervision, appearances, etc.): Click Here・Article updates are posted on X!
We stream experiment videos on the Science Neta Channel!
