The concept of energy was first observed by Gottfried Wilhelm Leibniz, a German philosopher in the 18th century. He called it "vis viva," or "living force." In the 19th century, Thomas Young, a physician, observed that wind could move sailboats without being a living creature. He coined the term "energy" from the Greek words meaning "a repository of work," which is still in use today.
The unit of energy in the International System of Units (SI) is the Joule, named after the English brewer James Prescott Joule. Joule discovered that it took 778 foot-pounds of work to heat one pound of water by one degree Fahrenheit (1 Btu) while brewing dark ale. This discovery established a relationship between kinetic and thermal energy. The traditional British unit of work is the foot-pound, which is the amount of work required to raise one pound weight on one foot against the force of gravity.
One foot-pound is equal to 1.356 joules, and one calorie (the amount of heat required to raise the temperature of one gram of water by one degree Celsius) is equal to 4.18 joules.
Use this calculator to convert energy units.