Quantum mechanics says measurement of observable always produces result that is one of eigenvalues of that observable. Subsequent measurement yields same value. For a particle in a box with infinite potential barriers if measure momentum doesn't that put system in eigenstate of momentum insuring subsequent same value of momentum. Doesn't this then violate uncertainty since know particle's postion with certainty of width of box. I don't understand how this doesn't violate uncertainty, in any event you could always measure momentum and with the particle in box it seems you can always violate uncertainty.