The Physics Behind Flight

Lesson 4

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Sir Isaac Newton

• English mathematician
• Physicist
• Astronomer
• Theologian
• Author (described in his time as a "natural philosopher")
• widely recognised as one of the most influential scientists of all time and as a key figure in the scientific revolution.

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Sir Isaac Newton

• Well known for his discoveries in optics (white light composition) and mathematics (calculus),
• But his formulation of the three laws of motion — the basic principles of modern physics — is what he is most famous for

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Three Laws Of Motion

Inertia

F = ma

Equal & Opposite

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Newton’s First Law: Inertia

Newton’s First Law:

• Every object in a state of motion tends to remain in that state of motion unless an external force is applied to it

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Newton’s First Law: Inertia

• An airplane would continue to move forward and upward if it were not for the opposing forces of drag and gravity working against its desired movement
• Therefore, in order for a plane to remain in flight it requires constant thrust from the engines to propel it forward and constant lift generated from the airfoil shape of its wings.

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Newton’s Second Law: F=ma

Newton’s Second Law:

• Acceleration is produced when a force acts on a mass

F

(FORCE)

=

m

(MASS)

x

a

(ACCELERATION)

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Newton’s Second Law: F=ma

• The greater the mass of the object being accelerated the greater the amount of force needed to accelerate the object
• For example, larger airplanes that have to carry more people or cargo must have more powerful engines to exert a greater force on the body of the airplane and get it to move forward and fly

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Newton’s Second Law: F=ma

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Newton’s Third Law: Equal & Opposite

Newton’s Third Law:

• For every action there is an equal and opposite reaction

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Newton’s Third Law: Equal & Opposite

• For example, as the propellers chew up the air in front of them to generate thrust, they are also shooting the used up air out the back side in the same quantity and at the same speed as it is being used

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NEWTON’s CRADLE

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Newton’s Third Law: Cause & Effect

• Another example can be seen by examining the air that moves over the airfoil shaped wing of an airplane

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• It comes off the end of the wing and shoots downward; the opposite reaction is that the air under the wing presses upward on the bottom of the wing and pushes the plane up

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LAW OF FLIGHT

The Law of Flight

Air particles �always travel �from high pressure to low pressure areas

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The Law of Flight

• For example:
• On a cold winter day when you are in a warm room the air pressure is low because the particles in warm air are spread farther apart than those in cold air

WARM AIR

COLD AIR

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The Law of Flight

• The air outside is cold and the air particles are much closer together and the pressure is greater.
• If you open a window… the cold air seems to come in very quickly and change the temperature of the room quite fast
• This is because the air molecules are moving from a crowded area (the cold outdoors) to a less crowded area (the warm room)

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BERNOULLI’S PRINCIPLE

Bernoulli’s Principle

• Bernoulli’s Principle:
• As the speed of a fluid increases, the pressure decreases.
• Air can be thought of as a fluid

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THE MORE WE STICK TOGETHER

PREDICT

OBSERVE

EXPLAIN

DEMO: Bernoulli’s Principle

X

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DEMO: Bernoulli’s Principle

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DEMO: Bernoulli’s Principle

FASTER FLOWING AIR

Bernoulli’s Principle:

As the speed of a fluid increases, the pressure decreases

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LOWER

AIR PRESSURE

HIGHER

AIR PRESSURE

HIGHER

AIR PRESSURE

Law of Flight: Air particles always travel �from high pressure to low pressure areas

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Bernoulli’s Principle

• Therefore, fast moving air has lower pressure than slow moving air
• Fast moving air creates areas of low pressure because the particles are spread farther apart while high pressure air has particles packed closer together.
• Remember as well that the Law of Flight states that air will always travel from high pressure areas to low pressure areas.

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Bernoulli’s Principle

• Bernoulli’s principle helps explain that an aircraft can achieve lift because of the shape of its wings
• They are shaped so that air flows faster over the top of the wing and slower underneath
• The high air pressure underneath the wings will therefore push up against the bottom of the wing in order to try and move to an area with lower pressure (top of the wing)
• The pushing up of the air particles on the bottom of the wing creates lift

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Bernoulli’s Principle

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How do planes fly?

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