(a) State the necessary conditions for momentum of a system to remain constant.
(b) State the necessary conditions for energy of a system to remain constant.
(c) Show the detailed steps and state the assumptions that allow Eq. 4.2.3 to be reduced to ∑ F = ma.
Eq. 4.2.3 : \sum{F}=\frac{D}{Dt}\int\limits_{sys}{V\rho dV}