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

concepts/mechanics/u3/m3-2-work-energy.tex

@@ -35,6 +35,8 @@ W_{\text{net}}=\Delta K=K_f-K_i=\tfrac12 mv_f^2-\tfrac12 mv_i^2.
 \]
 Thus the net work done on a body equals the change in its kinetic energy.}
 
+\wc{Net work is the algebraic sum}{Net work $W_{\mathrm{net}}=W_1+W_2+\cdots$ includes the \emph{signs} of individual works. If one force does $+120\,\mathrm{J}$ and another does $-24\,\mathrm{J}$, the net work is $+96\,\mathrm{J}$, \emph{not} $144\,\mathrm{J}$.}
+
 \nt{The work-energy theorem is often more efficient than combining Newton's second law with kinematics when the problem asks for a speed after a known displacement or after a known amount of work. It avoids solving for time and often avoids solving for acceleration explicitly. In AP mechanics, \emph{net work} means the algebraic sum of the work done by all forces on the chosen system. A force parallel to the displacement does positive work, a force opposite the displacement does negative work, and a force perpendicular to the displacement does zero work.}
 
 \pf{Short derivation from Newton II}{Let $m$ denote the constant mass of the body, let $\vec{v}$ denote its velocity, and let $d\vec{r}$ denote its infinitesimal displacement. Start with Newton's second law,