# Why is kinetic energy denoted by the letter $T$ in quantum mechanics?

Kinetic energy is often written as $$K$$, $$KE$$ or $$E_k$$. Where does $$T$$ come from in quantum mechanics? Why and how did it come to be different?

• I thought that it might be because it was close to $U$ and $V$ in the alphabetical order. It was just a speculation of mine, anyway. – Arch Stanton Mar 5 '18 at 16:00

I’m pretty sure Lagrange started this, in Méchanique analitique (1788, p. 224; 1809, p. 263; 1811, p. 311; 1815, p. 2): his predecessors mostly worked with the vis viva ($=2T$) instead.
(As to why he chose the letter $T$, no idea. Words like kinetic, energy, work, appeared only later.)
Gaspard-Gustave Coriolis and Jean-Victor Poncelet used the name "quantité de travail" (quantity of work) and "travail mécanique" (mechanical work) to denote the kinetic energy. I guess that this (the term "travail") could be the origin of the symbol $T$. See at the end of https://en.wikipedia.org/wiki/Vis_viva
• D. Bernoulli is mentioned in Wikipedia text, but not for using $T$. – Valter Moretti Mar 4 '18 at 9:05