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I've seen many derivations for the momentum operator, but I've a rather naive problem that I cannot figure out in the derivation done by Griffiths in "Introduction to Quantum Mechanics" book. In chapter 1, when he derives the momentum operator he states:

[tex]\frac{d <x> }{dt} = \frac{d}{dt}\int x |\psi (x,t)|^2 dx =

\int x \frac{\partial}{\partial t} |\psi (x,t)|^2 dx [/tex]

i.e. He assumed [tex]\frac{\partial}{\partial t} x = 0 [/tex]

Why did he do that? Is there any justification for it?

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# Quantum mechanics momentum operator

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