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Im confused by something that came up in my quantum mechanics lecture. The lecturer posed us a question. What result do I get for a measurement of momentum if I have already measured the energy.

I assumed after measuring energy that the system would be left in an energy eigenfunction. So I tried to do the momentum operator on this eigenfunction (the one with sin in) and I have something that looks like:

[i [itex]\hbar[/itex]Cos([itex]n \pi x/\sqrt{a}[/itex])]/a

Is that correct? If so how do I extract the eigenvalues from it?

Thanks

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# Measuring energy and then momentum

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