Rocket propulsion is a classic example of Conservation of Linear Momentum.

• The rocket and its fuel form an isolated system (initially at rest, $P=0$).
• When the engine ignites, hot gases are expelled backward at high velocity ($m_{gas}{\vec{v}}_{gas}$).
• To keep the total momentum zero, the rocket must gain an equal amount of momentum in the forward direction ($m_{rocket}{\vec{v}}_{rocket}$).
• This is also explained by Newton's Third Law: the rocket pushes the gas back, and the gas pushes the rocket forward (thrust).