A jet engine works on the principle of:

The law of conservation of linear momentum states that when no external force acts on a system, the total momentum of the system remains constant. Momentum is the product of mass and velocity (p = mv); it is a vector quantity, with both magnitude and direction.

This law follows from Newton's third law of motion, which states that to every action there is an equal and opposite reaction.

In a jet engine, air is drawn in at the front and compressed. Fuel is injected and burnt with this air in a combustion chamber, producing very hot, high-pressure gases. These gases are expelled backward at a very high speed through a nozzle at the rear. The gases gain a large backward momentum. Since the total momentum of the engine and the gases must remain conserved, the engine (and the aircraft) gains an equal momentum in the forward direction. This forward push is called thrust.

A rocket works on the same principle. However, a jet engine takes oxygen from the air for burning its fuel, so it can work only within the atmosphere. A rocket carries both its fuel and an oxidiser, so it can work in outer space, where there is no air.

Other examples of the conservation of momentum and Newton's third law:

Recoil of a gun: when a bullet is fired forward, the gun moves (recoils) backward. Because the gun is much heavier, its recoil velocity is much smaller than the bullet's velocity.

A swimmer pushes water backward to move forward.

When a person jumps from a boat to the shore, the boat moves away from the shore.

A balloon released with its mouth open flies in the opposite direction to the escaping air.

Why not the other options?

• ✗Conservation of energy states that energy can neither be created nor destroyed.
• ✗Bernoulli's principle explains the lift on aircraft wings, not the thrust of the engine.
• ✗Archimedes' principle explains buoyancy and floating.