A particle bound in an infinite potential wall at $x=0$ will apply a force on the wall. For a plane wave and imagining it as a fluid bouncing off the reflection wall at $x=0$, find the force in terms of $\phi'(0)$, the derivative of wave function at $x=0$. 

We know, for a reflecting fluid, $\textrm{force }= \frac{dp}{dt}$, where $p$ is momentum. Classically, $p=mv$. In our case, $v$ is $\hbar k/m$, the velocity of the plane wave. Instead of mass, we have $|\phi|^2dx$, the probability of finding the massive particle in an interval $dx$. So $p= \frac{\hbar k}{m}|\phi|^2 dx$. Then, force $f=\frac{dp}{dt} =\frac{d(|\phi|^2 dx \hbar k/m)}{dt}$

By the continuity equation, 
$\frac{d(|\phi|^2)}{dt}=-\frac{dj}{dx}$, where $j$ is probability current = $\hbar k/m |\phi|^2$.
So $f=\frac{dp}{dt}=\left(\frac{\hbar k}{m}\right)^2 \frac{d(|\phi|^2)}{dx} = \frac{\hbar k}{m} 2 \phi \phi' dx$. 

But for infinite potential, $\phi(0)=0$, so this gives 0. So I have an unwanted $\phi$ and a pesky $dx$ in my problem. I'm stumped for days, I'd appreciate all help. I'm fairly sure I should be getting an answer of force ~ $\phi'(0)^2$.


Question: Only showing the_date and the_excerpt for first entry in get_posts
Answer: I think you are having same issue as I was having earlier. In this question.

So here is the fixed code. This should work. I replaced both of these functions with get_the_date and get_the_excerpt. For the detailed explanation why/why not this work, read @kaiser's answer in same question.

&lt;?php
$args = array(
    'posts_per_page' =&gt; 5,
    'post_type' =&gt; 'my',  
    'order_by' =&gt; 'post_date',
    'tax_query' =&gt; array(
        array(
            'taxonomy' =&gt; 'mycategory',
            'field' =&gt; 'id',
            'terms' =&gt; array(36, 38, 83, 84),
            'operator' =&gt; 'NOT IN'
        ),
    ),
);
$last_five_posts = get_posts( $args );

foreach ( $last_five_posts as $post ) : setup_postdata( $post ); ?&gt;
    &lt;div&gt;&lt;a href="&lt;?php the_permalink() ?&gt;" rel="bookmark" title="Permanent Link to &lt;?php the_title_attribute(); ?&gt;" id="frontpagelatestposttitle"&gt;&lt;?php the_title(); ?&gt;&lt;/a&gt;&lt;/div&gt;
    &lt;div&gt;&lt;?php echo get_the_date(); ?&gt;&lt;/div&gt;
    &lt;div id="frontpagelatestpostexcerpt"&gt;
        &lt;p&gt;&lt;?php echo get_the_excerpt(); ?&gt;&lt;/p&gt;
    &lt;/div&gt;
&lt;?php endforeach;
wp_reset_postdata();
?&gt;