Energy is the capacity to do work or to produce heat. There are many forms of energy, each of which can be classified into three categories: radiant energy, kinetic energy, and potential energy. Energy can be changed, or transformed, from one form into another. Energy transformation is also called energy conversion. The Système International d'Unités (SI) unit for energy is the joule (J), after James Joule, who demonstrated that work can be converted into heat. Lifting a medium-sized potato a distance of 1 m (3.28 ft) would require approximately one joule of energy. Energy is often expressed as the calorie (cal), which is the amount of heat needed to raise the temperature of one gram of water by 1(C. One calorie is equal to 4.184 joules. The calorie (Cal), which is used to express the energy in food, is 1,000 calories.

Kinetic energy is the energy...