Aircraft move forward because their engines push air backward. This is a direct example of Newton's Third Law: when the aircraft exerts a backward force on air, the air exerts an equal and opposite forward force on the aircraft. That forward force is called thrust, and it is essential for takeoff, climbing, cruising, and overcoming drag.
The subtitle idea, Pushing Air Back to Move Forward, captures the central physics in one sentence.
A jet engine creates thrust by pulling in air, compressing it, mixing it with fuel, burning the mixture, and ejecting hot gas backward at high speed. A propeller creates thrust by spinning airfoil-shaped blades that accelerate a large mass of air backward, usually at a lower speed than a jet. In both cases, thrust depends on how much air is moved each second and how much its velocity changes.
A useful model is F = m_dot delta-v, where m_dot is the mass flow rate of air and delta-v is the change in air speed.
Key Facts
- Newton's Third Law: for every action force, there is an equal and opposite reaction force.
- Thrust is the forward force on an aircraft caused by accelerating air backward.
- Basic thrust model: F = m_dot delta-v.
- Mass flow rate is m_dot = mass of air moved per second, measured in kg/s.
- A jet engine usually gives a smaller mass of air a larger change in velocity.
- A propeller usually gives a larger mass of air a smaller change in velocity.
Vocabulary
- Thrust
- Thrust is the force that pushes an aircraft forward through the air.
- Newton's Third Law
- Newton's Third Law states that forces occur in equal and opposite pairs between interacting objects.
- Mass flow rate
- Mass flow rate is the amount of mass that passes through a system each second.
- Delta-v
- Delta-v is the change in velocity of an object or fluid, such as air moving through an engine.
- Propeller
- A propeller is a rotating set of airfoil-shaped blades that accelerates air backward to produce thrust.
Common Mistakes to Avoid
- Thinking the engine pushes against still air like a solid wall. Air is a fluid, and thrust comes from changing the momentum of moving air.
- Forgetting the direction of the reaction force. If the aircraft pushes air backward, the air pushes the aircraft forward.
- Using only air speed and ignoring mass flow rate. A small amount of very fast air and a large amount of slower air can both produce thrust depending on m_dot delta-v.
- Assuming propellers and jets use different laws of physics. Both create thrust by accelerating air backward and both obey Newton's Third Law.
Practice Questions
- 1 A small jet engine moves 20 kg of air each second and increases the air speed by 300 m/s. Using F = m_dot delta-v, what thrust does it produce?
- 2 A propeller accelerates 80 kg of air each second by 25 m/s. Calculate the thrust, then compare it to a jet that moves 10 kg of air each second by 200 m/s.
- 3 Two aircraft produce the same thrust. One uses a propeller that moves a large mass of air slowly, and the other uses a jet that moves a smaller mass of air very fast. Explain how both can satisfy F = m_dot delta-v and Newton's Third Law.