Properties

When something like an airplane or a boat go through air or water certain forces begin to show themselves. These forces need to be understood. A study of fluid (air or water) is necessary. In the Measurements section, units (inches, feet, meters) were introduced. These help you understand aerodynamic forces.

The properties of air/water can include temperature, pressure, density, viscosity, weight, gravity, velocity and acceleration. These subjects are further defined in this section.

Temperature

The temperature of a fluid is important. Hot oil, for example, flows faster than cold oil. warm air rises and cold air drops in a room. Notice that many homes have furnace vents in the floor. This is so the warm air will rise through the whole room. Very cold water is lighter than cool water. That is why a lake freezes from the surface down. You can see that knowing the temperature of a fluid can be important for aerodynamics. As mentioned in the Measurements section, temperature has units of degrees Fahrenheit (F) or degrees Celsius (C).

Pressure

The pressure of a fluid is important. As a fluid flows over or through an object it gives small pushes on the surface. A large surface with may small "pushes" can add up to a lot of pressure! Air pressure decreases (gets lower) at higher altitudes. Water pressure increases as you go deeper in the water. Pressures also decrease when the speed of the fluid increases. Warm air or water expands, thereby making more pressure. The pressures on a boat or airplane are very important to know and understand.

Density

Every "thing" has mass. This means molecules. Molecules are the smallest parts of something. Density is how many molecules in a "thing" and how closely packed together they are. Water has more density than air.

Some matter, like air, can be compressed (squeezed) into a smaller space, or spread out into a bigger space. Water cannot be compressed.

When pressure or temperature change, in a fluid, then density will change. Molecules will get closer together or farther apart.

Viscosity

Viscosity shows how fast or how slow a fluid will flow. If you spill water on an inclined board the water will run quickly down the board. However, if you spill honey on the same board it will travel down the board very slowly. Honey has a much higher viscosity than water.

Force

Forces are pushes or pulls on an object. These forces may be wind pressure, water pressure, weight or gravity. These forces may also be man-made, like a jet or rocket engine. But, for aerodynamics we study the air and water for these forces.

A force must have a direction. Look at the box in the picture. If you pushed it to the right it would move to the right. If you pushed down on the top of the box it would not move, because it is on the ground. Knowing the direction of a force is very important.

Weight and Gravity

Remember that mass means how many molecules are packed into a certain area. In other countries mass is measured in grams or kilograms. In the United States people use weight to also mean mass. This works okay at sea level because gravity is the same everywhere. If an object weighing 10 pounds is taken into the upper atmosphere the force of gravity is less and the object will weigh a little less (maybe 9.5 pounds). The mass, however, will stay the same. Scientists use units to separate mass and weight: pounds mass and pounds force (weight is force because of gravity).

The gravity on the moon is 1/6th less than on the earth. A sixty pound object on the earth would only weigh 10 pounds on the moon. The mass would stay the same.

Velocity

When you are riding in a car you know how fast you are going by looking at the speedometer. This is velocity. The units are miles per hour (mph) or kilometers per hour (kph). If you travel 120 miles in 2 hours your velocity is 60 mph (120 divided by 2).

Acceleration

When a car starts from zero mph and moves up to 60 mph this is called acceleration. If the vehicle stays at 60 mph for 10 minutes, there is no acceleration during that time. If a car is slowing down this is called deceleration.

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Last modified: Wed Jul 28 11:11:38 PDT 1999