Energy is the ability to do work. Energy may be stored (potential energy), or it can be said to be a property of an objects motion (kinetic energy). While the scientific definition of energy has been refined for centuries and thus has a very specific meaning, the mainstream usage of the word is often in conflict with the scientific definition. Additionally, the two classifications of energy (potential and kinetic) are often broken down into different types, leading some to incorrectly believe that there are other forms of energy. In fact, all forms of energy can be generalized as either potential or kinetic.
Potential energy is stored energy, that is, the amount of work that an object can do as a result of its position (and mass) relative to another object. For instance, a bowling ball that is held above the floor can be released, thus falling and crushing a pack of cigarettes on the floor. The bowling ball has potential energy because of its position relative to the floor (which gravity attracts it towards), and the work that it can do is the crushing of the pack of cigarettes. Not all types of potential energy are as easy to identify as the bowling ball, however. Potential energy is also the type of energy found in dry- and wet- cell batteries, nuclear reactions, and even springs.
Kinetic energy is often easier to identify. It is the result of the speed (and mass) of an object. For instance, the bowling ball from the previous example can be thrown at a glass window. The faster that the bowling ball is flying through the air, the greater the chance that the window will shatter. The destruction of the window is the work that is being done. Note a subtle difference between the potential energy discussed earlier, and this example of kinetic energy. In the previous example, the person releasing the bowling ball had to exert no effort to release the ball- in fact, she was relieved of the effort of holding the ball in place. However, the person throwing the ball at the window must in fact exert great effort to set the ball in motion. And the faster she throws the ball (thus increasing the chance that the window will break), the more effort she will have to exert. This effort comes as a result of her converting potential energy to kinetic energy in her muscles- mostly the breaking down of glucose which she acquired by eating. Other examples of kinetic energy are the flow of water through a turbine, wind blowing, and even electrons spinning around the nucleus of an atom.
Science Terminology Questions