In this situation, m1 represents the mass of the gas in the propulsion system, v1 represents the initial speed of this gas, m2 represents the mass of the rocket, and v2 represents the initial velocity of the rocket. On the other end of the equation, v1f represents the final velocity of the gas and v2f represents the final velocity of the rocket. Initially, both the gas in the propulsion system and the rocket are stationary, leading to v1 and v2 equaling 0.