Correction of a minor physics error (wrong variable)

exercices/newton.md

@@ -189,7 +189,7 @@ F-F_{BA}=F_A=\frac{m_AF}{m_A+m_B+m_C}.
 $$
 Il vient que
 $$
-F_{BA}=F-\frac{m_CF}{m_A+m_B+m_C}=\frac{(m_B+m_C)F}{m_A+m_B+m_C}=-F_{AB}.
+F_{BA}=F-\frac{m_AF}{m_A+m_B+m_C}=\frac{(m_B+m_C)F}{m_A+m_B+m_C}=-F_{AB}.
 $$
 5. Il suffit de remplacer. J'ai confiance en vous.