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TE Activity: Air Pressure

Air pressure is pushing on us all the time although we do not usually notice it. This activity will discuss the units of pressure and give the students a sense of just how much air pressure is pushing on them.

Engineers take into consideration their understanding of air pressure in the design of everything from airplanes to chemicals. The weight of the air is an important factor when developing the structure of aircraft and spacecraft. Environmental engineers pay careful attention to air pressure when designing wind turbines. Chemical engineers need to know how a chemical reacts in different air pressures. When designing anything that moves through the air, engineers analyze it to see how it reacts to the air pressure.

Learning Objectives
Materials
Introduction/Motivation
Procedure
Attachments
Assessment
Extensions
Activity Scaling
References

Grade Level: 5 (4-6)	Group Size: 1
Time Required: 30 minutes	Activity Dependency : None
Expendable Cost Per Group : US$ 0
Keywords: airplanes, pressure, altitude, air, atmosphere, weight, force, Bernoulli's Principle

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Colorado Math
- 1. represent, describe, and analyze patterns and relationships using tables, graphs, verbal rules, and standard algebraic notation; (Grades 5 - 8) [2005]
Colorado Science
- Standard 1: Students understand the processes of scientific investigation and design, conduct, communicate about, and evaluate such investigations. (Grades 0 - 12) [1995]
- 2.1 Students know that matter has characteristic properties, which are related to its composition and structure. (Grades 0 - 12) [1995]
- 4.2 Students know and understand the general characteristics of the atmosphere and fundamental processes of weather. (Grades 0 - 12) [1995]

Students should be able to:

Understand that the atmosphere exerts a pressure on objects.
Develop a sense for what 12 or 15 pounds per square inch actually feels like.
Understand that the pressure of the atmosphere will change depending on where it is being measured (e.g., Denver or Boston) due to differences in altitude.
Explain why they do not feel the pressure of the atmosphere pressing down on them.
Understand that the force on a surface caused by air pressure depends on the surface area.
Use algebraic methods to explore values of air pressure.
Measure in inches.
Work with sums and products of 12 and 15.
Recognize Patterns.
Analyze and interpret data using graphs.

Ask the students if they notice the pressure on their ears when they dive to the bottom of a swimming pool. The pressure in a pool increases with depth just as air pressure increases as you go deeper into the sea of air. The bottom of the sea of air is represented by sea level while higher elevations represent shallower parts of the atmosphere. As you move to shallower parts of the atmosphere, such as the top of Mount Everest, the pressure decreases.

Pressure is measured in different units. Scientists and engineers typically use the metric unit Pascal (Pa). A Pascal is defined as the pressure exerted by 1 Newton weight (1 kg under Earth's force of gravity) resting on an area of 1 square meter. Below is a list of some of the common units used to measure pressure and their equivalents. Please note that there are many other units that may be used.

At sea level, the atmospheric air pressure can be represented as any of the following: (Note: it is suggested that you write the units on the board.)

1.013 x 10⁵ Pa (Pascal or N/m²)
1 atm (atmosphere)
760 mm Hg (millimeters of mercury)
14.7 lb/in² (psi - pounds force per square inch) (if 1-pound weight rests on 1-square inch of surface area, the pressure is 1 psi)

Background

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. (To better understand this, hold a large book flat on your outstretched hand and to notice how much pressure the book puts on it. Next try to balance the book on the tip of your index finger. How much pressure does it seem to exert now?) It is also important to note that air pressure decreases with increasing altitude. It is helpful to think of the atmosphere as a swimming pool, and the water represents the air.

Before the Lesson

Make copies of the attached worksheets.

With the Students

In Denver, CO, the Earth's atmosphere has a force of about 12 pounds per square inch (psi). For reference, a gallon of milk or water weighs about 8 pounds. Have the students make a 1-inch by 1-inch square with their hands. Now ask the students what a 2x2 square looks like, and ask them how many pounds would be pressing down on that square. Solving for the force in Formula 1, the students can see that by multiplying the area (4 in²) by the pressure gives 48 pounds as an answer. (Note: multiply the length times the width to get the area of the square.)
Ask the students how many pounds would be pressing on a 3x3 square? (Answer: 108 lb for an area of 9 in².) A 4x4? (Answer: 192 lb for an area of 16 in².)
Have students complete the Air Pressure Worksheet.
Do the students see a pattern? What happens every time the area of the square increases by one in²? (Answer: The pounds of force increase by 12 for every one square inch increase in area. This is called a linear relationship. Linear means line, if you have the students make a line graph plotting the area versus the force, you will see that it makes a straight line. See the Air Pressure Worksheet for an example of the relationship between area and force.)

What happens every time the sides of the square are increased by one inch? (Answer: This is harder since the relationship is not linear. Every time you increase the length of the sides by one inch the force increases by more than 12 lbs. In fact, as the length of each side gets longer the increase in the force gets larger as well. When the length of the sides are one inch, the force is 12 lbs. If we increase the sides to two inches, the force becomes 48 lbs. This is an increase of 36 lbs. If we add another inch and make each side three inches, the force becomes 108 lbs, which is an increase of 60 lbs. If we plot the length of each side versus the force, we will see that the relationship is not linear. The line curves up, which is known as an exponential relationship.)

The average pressure on a middle school student is 24,000 pounds! Ask the students why they do not feel the 24,000 pounds, and why they are not crushed. (Answer: There is air inside the body from breathing, through the skin, ears, etc., and that air balances out the pressure on the outside of the body.)
The average force of the atmosphere at sea level is 15 lbs per square inch (almost two gallons of milk). Have the students repeat their calculations for the sea level pressure. (Cities to use: New York City, 87ft; San Diego, 3 ft; and Boston, 10ft. All are very close to sea level.)
Have students look at the Pressure vs. Altitude Graph and make pressure predictions for several places based on different altitudes (for example: 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), Airliner Cruising at 30,000 ft.) Have the students estimate what the air pressure would be one mile below sea level if there was no ocean (this number is not on the graph, which means the students will have to extend the line below the zero altitude line to estimate this number).

Pre-Activity Assessment

Discussion Question: Solicit, integrate and summarize student responses.

Ask students to review Bernoulli's Principle. Make sure everyone understands how the Bernoulli Principle relates to pressure. (The faster a fluid moves the less pressure it exerts.)

Activity Embedded Assessment

Worksheet: Have the students record measurements and follow along with the activity on their worksheet. After students have finished their worksheet, have them compare answers with their peers.

Post-Activity Assessment

Graphing: Have students use the information from their air pressure worksheet to create a line graph of the relationship between area and force. Area (in²) should go on the x-axis and force (ponds) should go on the y axis. Ask students or teams to explain what is happening in their graph in their own words.

Integrated Teaching and Learning Program, College of Engineering, University of Colorado at Boulder Tom Rutkowski, Alex Conner, Geoffrey Hill, Malinda Schaefer Zarske, Janet Yowell © 2004 by Regents of the University of Colorado.
The contents of this digital library curriculum were developed under a grant from the Fund for the Improvement of Postsecondary Education (FIPSE), U.S. Department of Education and National Science Foundation GK-12 grant no. 0226322. However, these contents do not necessarily represent the policies of the Department of Education or National Science Foundation, and you should not assume endorsement by the federal government.

Last Modified: April 27, 2006