content(warnings): Add W3-W5, N2-N3 — drag, work, and rolling warnings + notes

concepts/mechanics/u7/m7-3-shm-energy.tex

@@ -151,3 +151,5 @@ v=1.60\,\mathrm{m/s},
 \qquad
 v_{\max}=2.0\,\mathrm{m/s}\text{ at }x=0.
 \]
+
+\nt{In simple harmonic motion, total mechanical energy $E = K + U$ is constant. At maximum displacement ($x=\pm A$), $K=0$ and $U=E$ (all energy is potential). At equilibrium ($x=0$), $U=0$ and $K=E$ (all energy is kinetic). At $x=\pm A/\sqrt{2}$, $K=U=E/2$. The energy exchange oscillates at twice the frequency of the motion itself.}