How Does a Spinning Top Work?
The spinning top, usually a child's toy, is a fascinating glimpse into the physical laws that govern the universe. Energy, friction, velocity, momentum -- all come into play as the top spins wildly across a table or floor. But before this happens, the top is at rest yet possesses potential, or stored, energy. Until a force is exerted on it, it remains still, waiting for the action to happen. The shape of the top is important. An acorn is the perfect example of a top-shaped object: broad and rounded at the top, narrowing to a point at the bottom. However, until force is exerted upon it, it's just an acorn on the ground.
The top is steadied and rests with its bottom tip touching the floor. Most tops are put into motion by twisting the top stem with a snap and releasing. Other tops are spun by pulling a cord wrapped around their centers. Some tops are released from about hip level with a snap of the wrist and sent spinning to the floor. In any case, the potential energy within the top is released, changing to kinetic energy, the energy of movement and motion. The spinning top rotates around an unseen principal axis. If the top were perfect in every way -- perfectly balanced and weighted -- and the floor was perfectly level as well, the top would rotate pretty much indefinitely. But most tops aren't perfect and will begin to lose their momentum.
Because of the imperfection of the top, precession will occur, meaning that it will begin to change direction. If you've watched a spinning top, you may remember seeing it spin for quite a while in a nearly vertical fashion, like an Olympic ice skater performing a perfect spin. Eventually, predictably, the top begins to wobble ever so slightly, beginning to lean away from that straight up-and-down axis. It loses velocity and momentum, and before long is wobbling in ever-widening circles -- now looking like an inebriated Olympic ice skater -- until it finally flops over onto the floor and comes to a complete rest. Along with precession, the friction that is created between the top's tip and the floor is also at work to bring the perfectly spinning top down. Friction slows down any moving object.
- Illustration by Mary Osborne