What Energy Is

  • Energy is the ability of a system to perform work.
    • Energy cannot be measured directly, but when it changes form, it is called work, which can be measured.

The term work was first used by 19th century French engineer, Gaspard-Gustave Coriolis to simplify the concept of "weight lifted through a height." This was based on the use of steam engines that lifted buckets of water out of flooded mines. (Coriolis also coined the term "kinetic energy" for energy of motion.)

    • Work is the action of a force over a distance.
      • A force must be applied to an object to change its position.
      • By exerting a force over a distance, energy is converted into work.

Energy Weights & Measures:

The United States, along with 47 other industrial nations, uses a uniform system of weights and measures called The International System of Units, or SI.

SI consists of 7 base units and 22 units derived from the base units.


Following are the base units:

Unit Name Unit Symbol Quantity Name
meter m distance (length)
kilogram kg mass
second s time
ampere A electric current
Kelvin K temperature
mole mol amount of substance
candela cd intensity of light


Following are examples of common derived units:

Integration of System Units
Unit Name Unit Symbol Quantity Name
newton N force
joule J energy/work/heat
watt W power/radiant power


British thermal unit is another common term used with energy. It's abbreviated as "BTU" and equals about 1,055 joules. BTU is the amount of energy needed to cool or heat one pound of water by one degree Fahrenheit.       

Newton's Cradle
  • The Law of Conservation of Energy states that energy cannot be created or destroyed. The total amount of energy in the universe always has been and always will be the same. Energy can change from one form to another or be transferred from one object to another.