# Homework Help: Quantum mechanics, expectation value

1. Oct 25, 2011

### fluidistic

1. The problem statement, all variables and given/known data
Show that for the expectation values the following relations hold: $d \langle x \rangle /dt =\langle p \rangle /m$ and $d \langle p \rangle /dt = - \langle d V/dx \rangle$.

2. Relevant equations

$\langle x \rangle = \int _{- \infty}^{\infty} \Psi ^* x \Psi dx$.

3. The attempt at a solution
I'm trying to show the first relation. I think I have to do $\frac{d}{dt} \left [ \int _{- \infty}^{\infty} \Psi ^* x \Psi dx \right ]$. But I don't know how to evaluate the integral since Psi is unknown to me. I'm stuck here.

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