A pressing subject
Paul holds the atmosphere bar.
Make a steel bar that exerts the same pressure as the earth's atmosphere.
A steel bar, with a square cross section 1 inch (2.54 cm) on a side and at least 5 feet long.
Cut the steel bar to the correct length to exert 1 atmosphere of pressure.
The bar should exert a pressure of 14.7 pounds/inch2
Since out bar has a cross section of 1 square inch, then cut a piece of the bar with a weight of 14.7 pounds.
Out bar was 20 pounds and 73 inches long to start. we calculated that the bar weighed,
20 pounds/ 73 inches = 0.27 pounds/inch
We thus needed to cut it to a length of 54 inches or 135 cm.
(The atmospheric pressure at the surface of the earth varies by +- 6% or more so giving the answer to 2 significant figures is OK. or 1.4 m.)
To Do and Notice
Hold the bar in one hand.
Place the other hand on a table top.
Put the bottom end of the bar on top of the upturned palm of the hand on the tabletop.
Feel the force of the bar on your hand. Notice the pressure the bar exerts on one square inch of your flesh.
What's Going On?
The atmosphere of the earth reaches upward several miles above the surface of the earth. It is less dense than steel yet a column of air 1 square inch in cross section weighs 14.7 pounds.
Because the air is a gas, which is a fluid, it exerts a force inward on every square inch of your body. This bar allows you to feel the large size of this force.
Ever since you were born you have felt this force. Your body ignores sensations that remain constant over a long time, so you do not feel the pressure squeezing in on your body.
A bar that is 1/2 as long illustrates the pressure at 18,000 feet. A bar 1/3 as long shows the pressure at 26,000 feet.
A bar 0.6 % as long, just 0.8 cm, shows the pressure at the surface on Mars.
A bar 90 times as long, 125 meters high, shows the surface pressure on Venus. Such a bar would weigh over a half ton.
Ten meters, 30 feet, of water exerts a pressure equal to the entire atmosphere.
Half the atmosphere is below 5.6 km or 18,000 feet, a third of the atmosphere is below 8 km or 26,000 feet, 90% of the atmosphere is below 18 km or 54,000 feet or 10 miles, and 99% of the atmosphere is below 35 km or 100,000 ft.
Scientific Explorations with Paul Doherty
4 August 2004