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The Physics of Flight (Newton and Bernoulli)


(Newton and Bernoulli)


Have you ever looked into the sky, seen an airplane, helicopter or glider and wondered how is that possible? I know that metal is more dense than air, so how is it possible that something can fly? Basically it is all a question of force balance. If the upward force is greater than the gravitational force imposed on an object, the object will go up. If the gravitational force is greater, then the object will go down or remain on the ground. As with all things in nature, balance of force is a key component in just about every process. In flight there are 4 basic components:

  • Thrust
  • Drag
  • Lift
  • Weight

Newton’s First Law

“Every body remains in a state of rest or uniform motion unless it is acted upon by an external unbalanced force.”

If the thrust equals drag there is no change in the horizontal motion. If lift equals weight there is no change in the vertical motion. If any of these is increased or decrease so that the formula is unbalanced, a change will occur.


Newton’s Second Law

“A body of mass (m) subject to a force (F) undergoes an acceleration (a).”

F=m ×a

This law defines the amount of force, produced by lift, needed to overcome the effects of gravity. This lift is achieved in part by use of the Bernoulli principle, and also Newton’s Third Law of motion.

The Bernoulli Principle

The Bernoulli Principle simply states that an increase of a flow results in a decrease of pressure. When air hits the leading edge (rounded edge) of the airfoil as pictured above, the airflow above the wing travels faster than the flow below the wing. As the Bernoulli Principle states, the pressure below the wing will be higher than the air pressure above the wing. Because of this difference in pressure, lift is created. This lift pushes the wings upward. However this is not the only physics principle at work.


Newton’s Third Law of Motion

“For every action there is an equal and opposite reaction.”

It can be seen in the picture to the right that as the angle of attack is increased, as in the bottom picture, the resultant force from the deflection of the air both above and below the wing is also a major component to lift. As the air is deflected downward, as in the top picture, it pushes on the wing in an equal and opposite direction. Newton’s Third Law in conjunction with Bernoulli’s principle can be used to explain the physics behind lift that allows an airplane to fly. Lift combined with drag, weight, and thrust; provide the 4 forces which need to be controlled to allow an airplane to maintain flight.

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