Like almost any vehicle, the operation of a wind-up provides rich examples of how energy can be stored and converted from one form to another, but never created nor destroyed. This last principle is called the Law of Conservation of Energy. The energy transformations in a wind-up are summarized in the table above.
Let’s begin with the winding of the stick. It takes energy to do that. The energy of anything in motion is called kinetic or mechanical energy. Where do we get that energy from? Working backwards, all of the energy we have comes from food and beverages, which store chemical energy in a form that our bodies can make use of. Energy that is stored is available for use later, but not now. Another word for stored energy is potential energy. The energy stored in food or beverages originates from plants, even if we eat meat, whose energy came originally from the plants eaten by an animal. The chemical energy that is stored in plants is the result of photosynthesis, which converts solar (or light) energy from the sun into chemical energy. Ultimately, we are all solar-powered!
Next, let’s look forward in the process. What happens as we exert energy into winding the stick? The rubber band converts this energy into a form called elastic energy, which is the energy of something that is twisted, stretched or squashed, but “wants” to come back to its original shape. Elastic energy is a form of potential energy, because it is being saved up for later. The Homework is a scavenger hunt for devices other than wind-ups that store elastic energy, usually in some kind of spring. Energy is stored in the rubber band until the wind-up is released. At that point, as the rubber band turns the wheel through the paper clip, the stored energy is converted into the energy of motion, which we have already identified as kinetic or mechanical energy.
Eventually, the wind-up will stop rolling. What happens to its energy? When the wind-up stops, it has no kinetic energy any more, which seems to suggest that its energy was lost – but the Law of Conservation of Energy says that can’t happen. What actually happens is very subtle. The energy of the wind-up is not destroyed, but it is changed from kinetic energy to a form that cannot be used. What slows the wind-up down is friction, the enemy of all moving things.