Like almost any vehicles, the operation of wind-ups and balloon cars provide rich examples of how energy can be stored and converted from one form to another. The energy transformations in a wind-up are summarized in Table 1 below.
Let’s begin with the energy we put into wind-ups and balloon cars. We wind the stick of the wind-up with our hands and we push air into the balloons by our lungs. In both cases, our body is moving. 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 or inflating the balloon? As the rubber band or balloon stretches, it 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. Energy is stored in the rubber band or balloon until the wind-up or balloon car is released. At that point, the stored energy is converted into the energy of motion, which we have already identified as kinetic or mechanical energy.
Eventually, the wind-up or balloon car will stop rolling. What happens to its energy? When the wind-up or balloon car 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 is friction, the enemy of all moving things.