content(warnings): Add W6-W7, X2-X3, N4 — energy misconceptions + cross-refs

concepts/mechanics/u3/m3-3-potential-energy.tex

@@ -17,6 +17,8 @@ Thus the work done by the conservative force is
 W_c=-\Delta U.
 \]}
 
+\wc{Potential energy belongs to a system}{Gravitational $U=mgh$ requires both the object \emph{and} the Earth. Spring $U=\tfrac12 kx^2$ requires both the block \emph{and} the spring. An isolated single object cannot have potential energy --- it is stored in the \emph{interaction}.}
+
 \thm{Equivalent conservative-force relations}{Let $\vec{F}_c$ denote a conservative force and let $d\vec{r}$ denote an infinitesimal displacement. Then the following relations hold:
 \[
 \oint \vec{F}_c\cdot d\vec{r}=0,
@@ -146,3 +148,5 @@ W_s=+2.25\,\mathrm{J},
 \qquad
 v_f=3.0\,\mathrm{m/s}.
 \]
+
+The same spring restoring force $-kx$ governs the simple harmonic motion studied in Unit 7 (Sections 7.1--7.4).