A system possesses energy if it has the ability to do work.

Work shifts energy from one system to another.

Energy is …

Energy can exist in many different forms. All forms of energy are either kinetic or potential. The energy associated with motion is called kinetic energy. The energy associated with position is called potential energy. Potential energy is not "stored energy". Energy can be stored in motion just as well as it can be stored in position. Is kinetic energy "used up energy"?

kinetic energy

type motion examples and subtypes
motion of macroscopic objects machines, muscles, projectiles, wind, flowing water, ocean waves, sound, …
random motion of microscopic particles of matter (molecules, atoms, ions) heat, fire, geothermal, …
bulk flow of charges (electrons, protons, ions) household current, AC and DC circuits, lightning,…
disturbance propagating through electric and magnetic fields (classical physics) or the motion of photons (modern physics) radio waves, microwaves, infrared, light, ultraviolet, x‑rays, gamma rays
Types of kinetic energy (classified by type of object)

potential energy

force field quantity in field examples and subtypes
gravitational mass roller coaster, waterwheel, hydroelectric reservoir, …
electromagnetic charge electric, magnetic, chemical, elastic, …
strong nuclear color charge nuclear reactors, nuclear weapons, …
weak nuclear lepton number radioactive decay, …
Types of potential energy (classified by type of field)



English brewer and scientist James Joule (1818–1889) who determined the mechanical equivalent of heat.

J =  kg m2  =  N m
s2 s2
energy type object, phenomena, process, or event
700 nJ kinetic falling snowflake
85 µJ kinetic falling raindrop
3-12 J gravitational an apple in a tree
60 kJ heat making a cup of coffee or tea
270 kJ chemical an apple in the digestive tract
300 kJ kinetic car driving at freeway speed
1.5 MJ electrical television running for four hours
2 MJ chemical stick of dynamite
3.8 MJ chemical food for one person for one year
4.184 GJ chemical ton of TNT
63 TJ nuclear Hiroshima atomic bomb (Little Boy, 1945)
1.8 PJ kinetic Chelyabinsk meteor impact (2013)
42 PJ kinetic Tunguska meteor impact (1908)
240 PJ nuclear Largest nuclear bomb tested (Tsar Bomba, 1961)
Approximate energy of selected events


For those who want some proof that physicists are human, the proof is is the idiocy of all the different units which they use for measuring energy.

Richard Feynman, 1964

unit joule equivalent equivalent reference
joule 1 J   1 N m work
erg 0.1 μJ   1 dyne cm "
foot pound 1.35582 J (approximate)   "
watt second 1 J     power
watt hour 3.6 kJ   3600 W s "
kilowatt hour 3.6 MJ   1000 W h "
thermochemical calorie 4.184 J (by definition)   heat
International Table calorie 4.1868 J (by definition)   "
kilocalorie*       1000 calorie "
thermochemical Btu** 1.054350 kJ (approximate)   "
International Table Btu** 1.05505585262 kJ (by definition)   "
therm       100,000 Btu "
quad*** 1.055 EJ (approximate) 1015 Btu "
cubic meter natural gas 37–39 MJ (variable)   chemical
ton of oil equivalent 41–45 GJ (variable)   "
ton of coal equivalent 29.3 GJ (approximate)   "
ton of TNT 4.184 GJ (by definition)   "
Energy Units Discussed in This Book
* The kilocalorie is also known as the kilogram calorie, dietetic calorie, food calorie, and Calorie (with an uppercase "C").
** Btu is the abbreviation for "British thermal unit".
*** Quad is the shortened form of "quadrillion Btu".

Atomic and nuclear units