A Jet engine works on the principle of conservation of:

A jet engine operates on the principle of the conservation of linear momentum, which is a fundamental law of physics derived from Newton's third law of motion [1]. In a jet engine, air is taken in, compressed, heated, and then expelled at high velocity from the rear. According to the principle of conservation of linear momentum, the backward momentum of the expelled exhaust gases must be balanced by an equal and opposite forward momentum imparted to the engine [1]. This forward force is known as thrust. While jet engines also involve the conservation of mass (continuity equation) and energy (thermodynamics), the primary mechanical principle that explains the propulsion and force generation is the conservation of linear momentum. This ensures that the total momentum of the system remains constant in the absence of external forces [1].

Sources

1. [1] https://www.grc.nasa.gov/www/k-12/airplane/conmo.html