The Syrian airfield prepare for reception of military aircraft of China
The latest news about fast emergence in borders of Syria of navy of China encouraged Syrians and res[...]
Pressure
Pressure is the pressing effect of a force acting on a surface. Scientifically expressed, pressure is the force per unit area acting on a surface. Pressure (P) is defined as the force applied (F) divided by the area (A) of application. The equation for pressure is: P=F/A.
When a force acts on a material (solid, liquid, or gas), the result is pressure. The causes of pressure are as varied as the causes of forces. The force of a party balloon squeezing the air inside it produces pressure. The weight of a book pressing down onto a table produces pressure. Oil forced through the pipes of an aircraft’s hydraulic system produces pressure. Gravity pulling air against Earth’s surface produces atmospheric pressure.
Atmospheric Pressure
Atmospheric pressure, or air pressure, can be measured in various ways. The weight of air pressing down on the Earth’s surface produces an air pressure at sea level of 14.7 pounds per square inch (psi), or about 100 kilopascals (100,000 pascals). Meteorologists (weather scientists) measure pressure in bars. The air pressure at sea level is about 1 bar, or 1,000 millibars. This pressure also is known as “1 atmosphere.”
Air pressure in the atmosphere falls with increasing height. Gravity pulls air against Earth’s surface. Air at Earth’s surface has the weight of all the rest of
THE BAROMETER
Atmospheric pressure is measured with an instrument called a barometer. The first barometer was made in 1643 by an Italian scientist named Evangelista Torricelli (1608-1647). He filled a long glass tube with mercury. Then he turned the tube upside down with its open mouth in a bowl of mercury. Some of the mercury ran down into the bowl, but not all of it. A column of mercury about 30 inches (76 centimeters) high stayed in the tube. Its weight was balanced by air pressure acting on the mercury in the bowl. Torricelli realized that changes in the column’s level were due to changes in atmospheric pressure. Mercury barometers work in this way.
An aneroid barometer works in a different way. It is a sealed can with some air taken out. Atmospheric pressure squashes the can. The amount of squashing changes when the air pressure changes. These small movements are linked to a needle pointing at a press scale. Because they do not need a tall tube of mercury, aneroid barometers are much smaller than mercury barometers.
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the air above it bearing down on it, so the pressure is greatest here. Air higher in the atmosphere has less air from above pressing down on it, so the air pressure higher above the ground is lower. This is an important factor to consider for a person flying high in the atmosphere or going into space.
One-fifth of air, or about 20 percent, is oxygen. The thin, low-pressure air at the top of a high mountain contains the same percentage of oxygen as air near the ground, but because there is less air at high altitude, there is also less oxygen. The human body is very sensitive to sudden, even small, changes in pressure. Going up a tall building in a fast elevator can make someone’s ears pop. The shortage of oxygen in low-pressure air at high altitudes can cause more severe effects. When people go higher in the atmosphere, they may experience a variety of problems due to low air pressure.
Mountain climbers can suffer headaches, nausea, and dizziness when at altitude.