What is Closed system; momentum conservation?

A closed system exchanges no matter with the outside. For a system of particles, the internal forces cancel in pairs (third law), and the total momentum $\vect P = \sum m_i\vect v_i$ obeys $\dd\vect P/\dd t = \sum\vect F_{\mathrm{ext}}$: it is conserved when the external forces vanish (Chapter 19 develops this).