Measuring energy and then momentum

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Hi there,

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 $\hbar$Cos($n \pi x/\sqrt{a}$)]/a

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

Thanks

 

[mfb]

Eigenfunction where?
Your momentum measurement might depend on the position, if your potential is variable. A pure momentum measurement would not give a single, predictable value, but you could integrate over your whole wave function to get an expectation value.

 

[Hypersquare]

Sorry I meant in the well, where the potential is zero.