Under Pressure
What is Pressure?

Pressure (P) is defined as the amount of force
(F) applied per unit area (A) or as the ratio of
force to area:


P= F/A (Formula 1)
The pressure an object exerts can be calculated
if its weight (the force of gravity on an object)
and the contact surface area are known.

For a given force (or weight), the pressure it applies
increases as the contact area decreases.
Try this
 Place
your text book on your outstretched
hand. Notice how much pressure is being
exerted on your hand.
 Now try to hold the book on the tip of your
index finger. How much pressure does it
seem to exert now?
At the Bottom of an Ocean of Air


The atmosphere of our
planet is pressing down
on us all the time.
Air pressure is the
weight of the atmosphere
directly above us,
pushing down on our
heads and bodies.


The air pushes sideways,
too, at the same pressure.
The more molecules of air
there are, the more weight
presses on us.
An Inch of Air

Draw a square inch on a sheet of paper


Imagine a column of air one square inch in diameter
extending all the way to the top of the atmosphere
On earth at sea level the pressure is 14.7 pounds
per square inch (psi).

This is defined as one atmosphere (ATM).
You Explore
 With
your table see if you can determine
how many pounds are exerted on a given
square.
 Create a graph plotting area versus force
 Using your graph, what can you conclude
about the relationship between air
pressure and area?
Why Aren’t We Crushed
 The
average pressure on a middle school
student is 24,000 pounds!

What keeps the air from crushing our bodies?
• Remember the plastic bottle? What inside?
Is Air Pressure the Same
Everywhere?

Take a look at your pressure vs altitude graph
with your table. What can you conclude about
altitudes affect on the air pressure?
 Using your graph can you estimate what the air
pressure would be for the following locations.






Chicago, IL (580 ft)
Las Vegas, NV (2,030 ft)
Leadville, CO (10,177 ft)
Mt. Whitney, CA (14,495 ft)
Mt. Everest (29,035 ft)
Airplane Cruising at 30,000 ft.
How Air Pressure Works



The pressure is affected by two things:

How much atmosphere there is

The mass of the planet, which
determines the gravity that pulls the
air down.
Just like the pile of books

The pressure effect: the more books
there are, the more pressure there is
on the ones at the bottom.

The ones at the top feel very little
pressure.
If you are standing at a high elevation,
like on a mountain top, then there is less
atmosphere above you than if you are
standing at the beach (sea level).
Air Pressure on Other Planets

Venus


Same mass as the earth
More gas molecules in its
atmosphere.

Density = mass
volume
 At ground level, the
atmospheric pressure of
Venus is 90 atmospheres:

90 ATM x 14.7 lbs/inch2 =
1323 lbs/inch2.
Air Density

Because there is less air
pressure when we climb up
mountains the air is less dense
at higher altitudes



The molecules of air a farther
apart (some people describe this
as “thin air”)
We huff and puff, trying to get
enough oxygen into our lungs.
Our bodies are affected in two
ways
1. Reduced air pressure decreases
the amount of air we take in as we
breathe
2. There are fewer oxygen
molecules to breathe in each cm3
Measuring Air Pressure

Atmospheric pressure
is measured by an
instrument called a
barometer.


Mercury – air pressing
down on mercury force
it up glass tube
Aneroid – without
liquid
Reading Barometric Measurements
A
rising barometer indicates increasing air
pressure

Indicates good, dry, colder weather
A
falling barometer indicates decreasing
air pressure

Storms, rain and windy weather
Pressure Units
 Metric

A
unit for air pressure is a millibar
Average air pressure at sea level is 1013.25
millibars
line on a weather map connecting points
of equal air pressure is called and isobar
Homework
 Use
the terms on page 50 of your Science
Textbook to do the following:



Write a definition of each term
Write a sentence using each term
Draw a picture of each term.
 Read
article on Air Pressure and complete
worksheet.