Transcription
This kite surfer is hitching a ride on a moving airmass. Our transparent atmosphere reveals itself when it starts to move. Moving air, of course, is wind.
The atmosphere surrounding us is a deep ocean of air extending four kilometers above us. We know that the atmosphere is essential to life on Earth. The components of air, particularly oxygen and carbon dioxide, play a critical role in the chemistry of life. Another important aspect of air is atmospheric pressure. We live at the bottom of an ocean of air. Gravity pulls on this massive layer of gas creating pressure. The pressure is greatest at the bottom, the Earth's surface, the level where we live. Pressure is a force per unit area. The standard SI unit of pressure is the Pascal. One Pascal is one Newton of force per square meter. You will also find psi, or pounds per square inch, used in parts of the world, particularly America.
Air pressure at sea level is 14.7 PSI or 101.3 kiloPascals. That's over one hundred and one thousand Newtons of force per square meter. If you have a small kitchen table one meter square, the force on that area created by air pressure is approximately 101,000 Newtons, equivalent to the weight of ten cars sitting on the tabletop.
If air pressure can generate such huge forces, why don't we notice it? Why doesn't the table collapse? Turns out that pressure in a fluid, like air, transmits in all directions, not just down. This means that the 101,000 Newtons of force pushing down on the tabletop is balanced by 101,000 Newtons pushing up. Forces are balanced. Similarly, our bodies are not crushed because the fluids contained within our skin exert a pressure that balances atmospheric pressure.
Here is a simple demonstration that reveals the presence of atmospheric pressure. Fill a glass with water. Cut a small piece of card large enough to cover the top of the glass. Lay the card on the glass. Hold the card in place and rotate the glass full of water. Surprisingly, the water stays in the glass.
Here's the explanation: The card provides a rigid membrane between the water and the air in the room. Air pressure pushes up on the card. Air pressure is not pushing down on the water because there is a vacuum at the top of the water. Air pressure is pushing on the outside of the glass, but this does not affect the water. The only downward force from the water is its weight. The upward force of air pressure easily supports the weight of the water. If we drilled a hole in the top of the glass, air pressure would push on the top of the water; the water would fall from the glass.
The forces involved in our above demonstration are impressive. The glass I used in this demonstration has a diameter of 6 centimeters at the open end. The force acting on that small area is an incredible 286 Newtons, or 64 pounds, easily enough force to support the water in the glass. To confirm my numbers, calculate the area of the open end of the glass, then multiply by atmospheric pressure.
Interestingly, we couldn't live without atmospheric pressure. In the few minutes you have spent watching this video, your body has been quietly using air pressure to move air in and out of your lungs. The diaphragm, a muscular membrane below your lungs, alternately contracts and relaxes, changing the volume of the body cavity, which changes internal pressure. Low pressure allows air at atmospheric pressure to move down the trachea into the lungs. High pressure forces it out. Without atmospheric pressure, we could not breathe. Atmospheric pressure also plays a role in the function of many devices, including water pumps, syringes, and drinking straws. When you use a drinking straw, you're not sucking liquid up the tube; air pressure pushes the liquid up the tube.
For more videos about our atmosphere and other science topics, visit Hilah Road comm. Follow the videos link.