# Homework Help: Question 1.7 Griffiths Introduction to Quantum Mechanics (2nd Edition)

1. Sep 12, 2011

### aschulz90

Calculate d<p>/dt Answer: d<p>/dt = <-dV/dx>

generally speaking (I believe) you need to use scrodinger's equation (both of dphi/dt and dphi*/dt)
and the expectation value of momentum:

<p> = -i*h_bar * integral (phi* * dphi/dx) dx.

I would say that the way I used Cramster was just short of plagiarism because I really don't know what I'm doing with this problem. I don't believe we are supposed to use Hamiltonian to prove this problem because the book won't get to them 'till chapter 2.

The proof took about 1.5 pages and didn't even seem conclusive at the end. Could someone please just provide a general explanation of how this was solved?

I used Cramster already to answer this question (odd number so it's free to view with an account):
http://www.cramster.com/solution/solution/195547

Last edited by a moderator: Apr 26, 2017
2. Sep 12, 2011

### jncarter

My quantum mechanics professor assigned this problem last year, it requires some fancy algebra.
Use the Schroedinger equation to find the expectation value of $frac{dV}{dx}$. You'll have to get a bit creative and it involves using intergration by parts (if I remember how I did this problem).
You will also have to find the expectation value of p, then take the time derivative.

Next time try going to the professor for help before looking up the answer. You'll learn more.