Now that we have a better understanding of Air Vehicles (AVs) and the external forces acting upon them, let’s delve into the scientific principles associated with flight and how lift is actually attained.

### Newton’s Laws of Motion

Sir Isaac Newton, a physicist and mathematician, is one of the most influential scientists in history. Observing apples fall from a tree, Newton discovered gravity and formulated the **Laws of Motion**. These three laws are the foundation for studying classical mechanics and help explain the theory of lift.

#### Newton’s First Law

Newton’s First Law states:

"An object at rest stays at rest and an object in motion stays in motion with the same speed and in the same direction unless acted upon by an unbalanced force.”

This law, often described as the definition of inertia, is exemplified by an AV in motion. A cruising AV flies at a constant airspeed, meaning thrust exactly balances the drag. With no change in net forces, the AV travels at a constant velocity in a straight line. If thrust increases, creating an unbalanced force, the AV speeds up.

#### Newton’s Second Law

Newton’s Second Law states:

"Acceleration is produced when a force acts on a mass. The greater the mass (of the object being accelerated), the greater the amount of force needed (to accelerate the object).”

In flight terms, this means that a larger AV requires more thrust to move the same distance as a smaller AV. This law is expressed by the equation ( F = MA ) (Force equals Mass times Acceleration). Using this equation, we can compute an AV’s speed, weight, and thrust characteristics.

#### Newton’s Third Law

Newton’s Third Law states:

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

This law is crucial for flight, as it underpins the principle of lift. As an AV is thrust forward, the air is deflected downward by the airfoil, and in reaction, the wing is pushed upward.

### Bernoulli’s Principle of Lift

Bernoulli’s Principle states that as the velocity of a fluid increases, the pressure within the fluid decreases.

An airfoil (wing, rotor blade, or propeller) produces lift by increasing the distance the air travels over the top surface. Because the air has a farther distance to travel in the same amount of time, it must move at a higher velocity over the top. This higher velocity results in lower pressure on the top surface compared to the bottom, creating lift.