Let's say there are two observables corresponding to two operators A and B, and let's say my system is in a state Phi where with probability 1 if I measure A I get 3 (let's say 3 Joules), If I measure B I get 4 (let's say 4 m/s). If I measure A and then B I would get 3 Joules for that energy measurement and 4 m/s for the speed measurement, however, mathematically, I would write:
BA Phi=12 Phi
So the measurements kind of mixed up, I don't understand this.
This question arised from problem 3.5 of Zetilli's book