Energy is defined as the capacity to do work. The English physicist and physician Thomas Young (1773-1829) was the first person to use the term energy in this sense. The concept of energy unites almost all branches of science through its various manifestations (light, heat, atomic and subatomic behavior, etc.). Energy can be converted from one form to another and the total energy in any closed system remains constant. In classical physics, this principle was known as conservation of energy; in modern physics, it is termed the conservation of mass and energy.

In classical physics, there are two types of energy: kinetic and potential. (Modern physics recognizes a third type, rest-mass energy.) When two moving objects collide, the first object may bounce off the second in a new direction but with no change in speed, or it may slow down, or it may be...