4 basics every engineer must know:

What pressure is and how it works is so fundamental to the understanding of aerodynamics.

There are two ways to look at pressure:

Action of individual air molecules(using kinetic theory of gases) .

The action of a large number of molecules

By definition, Pressure is defined as the normal force per unit area.

$P=\frac{F}{A}$**Unit of pressure is N/**m** ^{2 }**,

**Bar, Pascal.**

Pressure is a **Scalar** quantity (i.e. it has only magnitude, not the direction)

A gas is composed of a large number of molecules that are very small relative to the distance between molecules.The molecules of a gas are in constant, random motion and frequently collide with each other and with the walls of any container.The molecules possess the physical properties of mass, momentum, and energy.

The momentum of a single molecule is the product of its mass and velocity,

Momentum P=m×v

As the gas molecules collide with the walls of a container, the molecules impart momentum to the walls, producing a force perpendicular to the wall.

The sum of the forces of all the molecules striking the wall divided by the area of the wall is defined to be the pressure.

The pressure of a gas is then a measure of the average linear momentum of the moving molecules of a gas. The pressure acts perpendicular (normal) to the wall.

The tangential (shear) component of the force is the shear stress.

If the gas as a whole is moving, the measured pressure is different in the direction of the motion. The ordered motion of the gas produces an ordered component of the momentum in the direction of the motion. We associate an additional pressure component, called **dynamic pressure**, with this fluid momentum.

Dynamic pressure

$q=\frac{1}{2}\rho {\nu}^{2}$The pressure measured in the direction of the motion is called the **total pressure** and is equal to the sum of the static and dynamic pressures described by Bernoulli’s equation.

An important property of any gas is its density. Understanding density and how it works is fundamental to the understanding of rocket aerodynamics.

Density is defined as the mass of an object per unit volume. We know that some objects are heavier than other objects, even though they are the same size.

Density is a **scalar** quantity. Its unit is kg/m^{3}

Different materials have different density. For example aluminum is less dense than iron. That is why airplanes, rockets, and some automobile parts are made from aluminum. For the same volume of material, one metal weighs less than another does if it has a lower density.

For solids, the density remains constant because the molecules are tightly bound. For example, a pure silver coin on the earth weighs same as in the moon.

However, for gases, the density can vary over a wide range because the molecules are free to move. Air at the sea level is different from the air at the stratosphere.

A gas is composed of a large number of molecules that are very small relative to the distance between molecules. The molecules are in constant, random motion and frequently collide with each other and with the walls of a container. Because the molecules are in motion, a gas will expand to fill the container. Density depends directly on the size of the container in which a fixed mass of gas is confined.

As a simple example, consider the figure. We have 10 molecules of a mythical gas. Each molecule has a mass of 50 grams (.05 kilograms), so the mass of this gas is .05 kg. We have confined this gas in a rectangular tube that is 1 meter on each side and 3 meters high. We are viewing the tube from the front, so the dimension into the slide is 1 meter for all the cases considered. The volume of the tube is 3 cubic meters, so the density is .16 kg/cubic meter.

An important property of any gas is **temperature**. An entire branch of physics, called thermodynamics, is devoted to studying the temperature of objects and the transfer of heat between objects of different temperatures.

The molecules are in constant, random motion and frequently collide with each other and with the walls of any container. The molecules possess the physical properties of mass, momentum, and energy. The momentum of a single molecule is the product of its mass and velocity, while the kinetic energy is one half the mass times the square of the velocity.

Unit is K, degree Celsius and degree farenheit.

The **speed of “sound”** is actually the speed of transmission of a small disturbance through a medium. The sound itself is a sensation created in the human brain in response to sensory inputs from the inner ear.

Disturbances are transmitted through a gas as a result of collisions between the randomly moving molecules in the gas.

The conditions in the gas are the same before and after the disturbance passes through. Because the speed of transmission depends on molecular collisions, the speed of sound depends on the state of the gas.

The speed of sound is a constant within a given gas and the value of the constant depends on the type of gas (air, pure oxygen, carbon dioxide, etc.) and the temperature of the gas.

Speed of sound =

$\sqrt{\gamma}RT$These 4 basic definitions play a strong role in every sort of science. there is no need of memorising these definitions, but understanding provides good foundation for learning. These values vary from region to region in the atmosphere.