Final answer:
According to the Ehrenfest theorem, the expectation value of position for a particle in a time independent potential and not in any superposition of eigenstates is time-independent. However, the expectation value of momentum can have nonzero values depending on the uncertainty in the particle's position.
Step-by-step explanation:
According to the Ehrenfest theorem, for a particle in a time independent potential and not in any superposition of eigenstates, the expectation value of position is indeed time-independent.
However, the expectation value of momentum is not always zero. The uncertainty principle in quantum mechanics states that if the uncertainty on the particle's position is small, the uncertainty on its momentum is large, and vice versa. Therefore, the expectation value of momentum can have nonzero values depending on the uncertainty in the particle's position.