Engine Thrust
Revised 01-Mar-2001
Back To Index

A jet engine does not create a thrust force by pushing exhaust gases against atmospheric air. It is a reaction engine that increases the energy of the gases that go through it to produce thrust.

It can do this courtesy of Isaac Newton's second and third law of motion (guy with the apple). To recap those laws:

Second Law: Force = Mass x Acceleration (F=Ma). A force is produced when a mass of air is accelerated. This also means that a force is produced when the momentum of the air is increased.
Third Law: For every force or action there is an equal or opposite reaction.

A jet engine increases the momentum of the air that goes through it. This can be stated as
Force = Momentum of the exhaust gases minus the momentum of the incoming air.
The engine applies a rearward force to the air which is equal to the increase in momentum Due to the third law the engine is pushed forward by a equal reactive force.

We can show the above in a practical way, we can calculate the thrust produced by an engine as follows. Add together all the positive forces on the engine internal surfaces. Add together all the negative forces on the engine internal surfaces.

The net thrust is equal to total forward forces minus total rearward forces. See below for an example.

Back To Index