A jet airplane obviously has a force that propels it forward during takeoff. Likewise for the rocket being launched in FIGURE 5.10. This force, called thrust, occurs when a jet or rocket engine expels gas molecules at high speed. Thrust is a contact force, with the exhaust gas being the agent that pushes on the engine. The process by which thrust is generated is rather subtle, and we will postpone a full discussion until we study Newton’s third law in Chapter 7. For now, we will treat thrust as a force opposite the direction in which the exhaust gas is expelled. There’s no special symbol for thrust, so we will call it $\vec{F}{}_{\text{thrust}}$.

FIGURE 5.10 Thrust force on a rocket.