How Does It Work
The Spinning Penny is scientific poetry in motion. To understand how and why it works, you have to look at the forces that are acting on the penny (object). Of course, you add the energy to get things started and to keep it going with the force you put in to swirling the balloon. The shape of the balloon forces the penny (or any object in it) to move in a circular path – otherwise, it would continue to move off in a straight line. Another force to consider is friction between the balloon and the object. While there’s very little friction between the edge of the object and the balloon, it is still there and doing its job. Friction finally causes the penny to slow down and to stop. It gets lower and lower inside the balloon because of our old friend, gravity, too. While the object’s mass stays the same, its speed drops because of friction against the balloon and moves lower because of gravity.
The real force in action here is called centripetal force, which means center-seeking. This is a force that is always directed toward the center of circular movement and is actually responsible for keeping the penny moving in a circle. Inside the balloon, it’s the wall of the balloon that causes this to happen. Out in the solar system, it’s the pull of the Sun’s gravity that keeps Earth in its circular orbital path.
Hopefully, you used a bumpy-sided object (like a hex nut) and heard some crazy noises. It sounded like the balloon was screaming! The rough sides of the object don’t roll smoothly over the balloon wall. Instead, it bounces from one flat edge to another. This bouncing accounts accounts for the noise and the change in pitch comes from the speed of the object: higher pitch = faster speed and lower pitch = slower speed.
