What is your favorite physics equation and why?

[will022]

I’m curious to know what is everyone’s favorite physics (or closely related field) equation, and the reason for being a favorite. It could be because you think the equation is profound, historical, beautiful, one you frequently use, etc.

Optional bonus question: what is your favorite way of writing/presenting your favorite equation?

[will022]

i am still waiting for the answers.

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Edited November 17, 2020 by will022

[papu]

Schrödinger equation by far, the way it's constructed, the history behind it, it's polar coordinate version and the hydrogen atom solution.

[JJMarchetta]

f=ma

[Jacko-Dark]

Time evolution in Heisenberg picture