We have some experience with gas pressure that we don't have with properties like viscosity and compressibility. Every day we hear the TV meteorologist give value of the barometric pressure of the atmosphere And most of us have blown up a balloon or used a pump to inflate a bicycle tire or a basketball.
Because understanding what pressure is and how it works is so fundamental to the understanding of aerodynamics, we are including several slides on gas pressure in the Beginner's Guide. An interactive atmosphere simulator allows you to study how static air pressure changes with altitude.
The FoilSim program shows you how the pressure varies around a lifting wing, and the EngineSim program shows how the pressure changes through a turbine engine. Another simulator helps you study how pressure changes across shock waves that occur at high speeds.
There are two ways to look at pressure: 1 the small scale action of individual air molecules or 2 the large scale action of a large number of molecules. From the kinetic theory of gases, 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, while the kinetic energy is one half the mass times the square of the velocity. No wonder the tank must be strong. The force exerted on the end of the tank is perpendicular to its inside surface. This direction is because the force is exerted by a static or stationary fluid.
We have already seen that fluids cannot withstand shearing sideways forces; they cannot exert shearing forces, either. Fluid pressure has no direction, being a scalar quantity. The forces due to pressure have well-defined directions: they are always exerted perpendicular to any surface. See the tire in Figure 2, for example. Finally, note that pressure is exerted on all surfaces.
Swimmers, as well as the tire, feel pressure on all sides. See Figure 3. Figure 2. Pressure inside this tire exerts forces perpendicular to all surfaces it contacts. The arrows give representative directions and magnitudes of the forces exerted at various points. Note that static fluids do not exert shearing forces.
Figure 3. Pressure is exerted on all sides of this swimmer, since the water would flow into the space he occupies if he were not there. Atmospheric pressure drops as altitude increases. The atmospheric pressure on Denali, Alaska, is about half that of Honolulu, Hawai'i. Honolulu is a city at sea level.
As the pressure decreases, the amount of oxygen available to breathe also decreases. At very high altitudes, atmospheric pressure and available oxygen get so low that people can become sick and even die.
Mountain climbers use bottled oxygen when they ascend very high peaks. They also take time to get used to the altitude because quickly moving from higher pressure to lower pressure can cause decompression sickness. Decompression sickness, also called "the bends", is also a problem for scuba divers who come to the surface too quickly. Aircraft create artificial pressure in the cabin so passengers remain comfortable while flying.
Atmospheric pressure is an indicator of weather. When a low-pressure system moves into an area, it usually leads to cloud iness, wind , and precipitation.
High-pressure system s usually lead to fair, calm weather. As you go up in an airplane, the atmospheric pressure becomes lower than the pressure of the air inside your ears. Your ears pop because they are trying to equalize, or match, the pressure. The same thing happens when the plane is on the way down and your ears have to adjust to a higher atmospheric pressure. Also called standard atmospheric pressure. Also known as DCS, divers disease, and the bends.
High-pressure systems are usually associated with clear weather. Low-pressure systems are often associated with storms. This is because your shoes have a small surface area. Your weight is only spread out over a small area, so the pressure on the snow is high.
However, you will not sink so far into the snow if you are on skis. This is because your weight is spread out over a greater surface area, so the pressure on the snow is low. Drawing pins have a large round end for your thumb to push.
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