Energy is "capacity (or ability) to do work," and work is "the result of a force acting through some distance." A car running into the rear of a stalled car exerts a force on it, pushing it some distance, doing work in the process. The capacity of the moving car to do work is termed its kinetic energy. The greater a car's speed and/or mass, the greater its capacity to do work—that is, the greater its kinetic energy.
Formally, the kinetic energy (K) of a mass (m) moving with speed (v) is defined as K=1/2 mv2. Kinetic energy is measured in joules (J) when m and v are expressed in kilograms (kg) and meters per second (m/s). A 1,000-kg car traveling 15 m/s (about 30 miles per hour) has 112,500 J of kinetic energy. Kinetic energy depends much more on speed than on mass. That is because doubling the mass of an object doubles the kinetic energy, but doubling the speed quadruples the kinetic energy. A 4,000-kg tractor trailer traveling at 30 m/s has the same kinetic energy as a 1,000-kg car traveling at 60 m/s.
Invariably, energy of use to a society is kinetic. A car is useful when it is in motion. Water in motion is useful for driving turbines in a hydroelectric plant. Electricity, the most versatile of all forms of energy, involves electric charges (electrons) in motion. And thermal energy, which provides energy for a steam turbine, is associated with the kinetic energy of molecules.
Most energy converters convert sources of potential energy to forms of kinetic energy that are useful. Gasoline in the tank of an automobile has potential energy. When burned, potential energy is converted to heat, a form of kinetic energy. Uranium in the core of a nuclear reactor has nuclear potential energy. Conversion of nuclear potential energy through nuclear fission reactions produces nuclei and neutrons with kinetic energy. This kinetic energy is the source for the electric energy produced by the nuclear power plant. Water atop a dam has gravitational potential energy. Flowing toward the bottom of the dam, the water continually loses potential energy but gains kinetic energy (and speed). The potential energy the water had at the top of the dam is converted entirely to kinetic energy at the bottom of the dam.
Every molecule in the air around you has kinetic energy because each has mass and is in incessant motion. According to the kinetic theory of gases, each of the molecules has average kinetic energy given by E=3/2 kT where k is the Boltzmann constant, having a value of 1.38 x 10-23 joules per Kelvin, and T is the Kelvin temperature. Interestingly, the average kinetic energy is independent of the mass of the molecule. Thus, nitrogen molecules and oxygen molecules in the air we breathe have the same average kinetic energy. On the other hand, their average speeds differ because their masses differ. When considering the fundamental meaning of temperature, it is appropriate to think of temperature as measuring the average kinetic energy of a molecule in a gas.
Flywheels of reasonable size and speed can store energy comparable to that of batteries and have promise for storing energy for electric vehicles. When a car is brought to rest by braking, the kinetic energy of the car is converted into heat that dissipates into the environment. Conceivably, the car could be brought to rest by transferring the linear kinetic energy of the car to rotational energy in a flywheel, with very little lost as heat during the conversion. The rotational energy then could be recovered by allowing the flywheel to operate an electric generator.
Hobson, A. (1995). Physics: Concepts and Connections. Englewood Cliffs, NJ: Prentice-Hall.
Priest, J. (2000). Energy: Principles, Problems, Alternatives, 5th ed. Dubuque, IA: Kendall/Hunt Publishing Company.
Serway, R. A. (1998). Principles of Physics, 2nd ed. Fort Worth: Saunders College Publishing.