Newton's Third Law explains how rockets work: fuel ejected from the nozzle exerts an equal and opposite force on the rocket, causing it to accelerate upwards. Specific impulse measures a rocket engine's efficiency - how fast it ejects fuel for a given amount of thrust. Higher specific impulse means less fuel is needed. The nozzle helps increase fuel velocity and specific impulse using the continuity equation, but must be carefully designed to avoid instability from overexpansion or loss of efficiency from underexpansion. As a rocket burns fuel, its mass ratio - initial to final mass - increases, improving acceleration according to Newton's Second Law. Forces acting on a rocket include weight, drag, thrust, lift, and gravity.