A book exerpt in Saturday's Globe and Mail seemed to me to have an
incorrect story about kinetic energy. However, with the help of the good
old 11th edition of the Encyclopaedia Britannica I unearthed corroboration
of the newspaper story, which ended in 1749 and I was able to trace the
history to the present day.
Isaac Newton called mv "quantity of motion", which is now known a momentum.
Leibnitz argued that quantity of motion would be lost in collisions, but
that mv^2, which he called "vis viva", would be preserved. I think that in
modern language we would say that mv is a vector quantity, whereas mv^2 is
a scalar.
Willem 'sGravesande dropped small brass spheres onto a soft clay bed and
measured the depth each ball penetrated. He found that if he doubled the
speed of impact the ball would penetrate four times as deeply, and if the
sphere hit the clay three times as fast it would penetrate nine times as
deeply.
A brilliant Frenchwoman, Emilie du Ch�telet, (1709 - 1749), saw that
'sGravesande's experiment was the experimental proof of Leibnitz's
hypothesis.
Her work was picked up by Lagrange, (1736 - 1813).
The name "energy" was applied to mv^2 by Young (1773 - 1829). Finally,
Rankine (1820 - 1872) named mv^2/2 as "actual energy".
I suspect that practical interest in measuring energy began with James Watt
when he decided to sell his steam engines on the basis of horsepower, one
horsepower = 550 foot-pounds per second. There has never been a single
English name for a unit of energy. Physicists don't like the pound-force
so British physicists for a time used foot-poundals for energy. We only
find a proper name for a unit of energy in the metric system. The Shorter
Oxford Dictionary places the first use of "erg" in English at 1873 and the
"joule" at 1882. Le Petit Robert places the first use of "erg" in French
at 1890, and does not give a date for the "joule".
