Under PressureUnder Pressure

What is Pressure?What is Pressure?

Pressure (P)Pressure (P) is defined as the amount of force is defined as the amount of force (F) applied per unit area (A) or as the ratio of (F) applied per unit area (A) or as the ratio of force to area: force to area: P= F/AP= F/A (Formula 1) (Formula 1)

The pressure an object exerts can be calculated The pressure an object exerts can be calculated if its weight (the force of gravity on an object) if its weight (the force of gravity on an object) and the contact surface area are known. and the contact surface area are known. For a given force (or weight), the pressure it applies For a given force (or weight), the pressure it applies

increases as the contact area decreases. increases as the contact area decreases.

Try thisTry this

Place your text book on your outstretched Place your text book on your outstretched hand. Notice how much pressure is being hand. Notice how much pressure is being exerted on your hand.exerted on your hand.

Now try to hold the book on the tip of your Now try to hold the book on the tip of your index finger. How much pressure does it index finger. How much pressure does it seem to exert now?seem to exert now?

At the Bottom of an Ocean of AirAt the Bottom of an Ocean of Air

The atmosphere of our The atmosphere of our planet is pressing down planet is pressing down on us all the time. on us all the time.

Air pressureAir pressure is the is the weight of the atmosphere weight of the atmosphere directly above us, directly above us, pushing down on our pushing down on our heads and bodies. heads and bodies.

The air pushes sideways, The air pushes sideways, too, at the same pressure. too, at the same pressure.

The more molecules of air The more molecules of air there are, the more weight there are, the more weight presses on us. presses on us.

An Inch of AirAn Inch of Air Draw a square inch on a sheet of paperDraw a square inch on a sheet of paper

Imagine a column of air one square inch in diameter Imagine a column of air one square inch in diameter extending all the way to the top of the atmosphereextending all the way to the top of the atmosphere

On earth at sea level the pressure is 14.7 pounds On earth at sea level the pressure is 14.7 pounds per square inch (psi). per square inch (psi). This is defined as This is defined as one atmosphere (ATM).one atmosphere (ATM).

You ExploreYou Explore

With your table see if you can determine With your table see if you can determine how many pounds are exerted on a given how many pounds are exerted on a given square.square.

Create a graph plotting area versus forceCreate a graph plotting area versus force Using your graph, what can you conclude Using your graph, what can you conclude

about the relationship between air about the relationship between air pressure and area?pressure and area?

Why Aren’t We CrushedWhy Aren’t We Crushed

The average pressure on a middle school The average pressure on a middle school student is 24,000 pounds! student is 24,000 pounds! What keeps the air from crushing our bodies?What keeps the air from crushing our bodies?

• Remember the plastic bottle? What inside?Remember the plastic bottle? What inside?

Is Air Pressure the Same Is Air Pressure the Same Everywhere?Everywhere?

Take a look at your pressure vs altitude graph Take a look at your pressure vs altitude graph with your table. What can you conclude about with your table. What can you conclude about altitudes affect on the air pressure?altitudes affect on the air pressure?

Using your graph can you estimate what the air Using your graph can you estimate what the air pressure would be for the following locations.pressure would be for the following locations. Chicago, IL (580 ft)Chicago, IL (580 ft) Las Vegas, NV (2,030 ft)Las Vegas, NV (2,030 ft) Leadville, CO (10,177 ft)Leadville, CO (10,177 ft) Mt. Whitney, CA (14,495 ft)Mt. Whitney, CA (14,495 ft) Mt. Everest (29,035 ft)Mt. Everest (29,035 ft) Airplane Cruising at 30,000 ft. Airplane Cruising at 30,000 ft.

How Air Pressure WorksHow Air Pressure Works The pressure is affected by two things:The pressure is affected by two things:

How much atmosphere there isHow much atmosphere there is The mass of the planet, which The mass of the planet, which

determines the gravity that pulls the determines the gravity that pulls the air down. air down.

Just like the pile of books Just like the pile of books The pressure effect: the more books The pressure effect: the more books

there are, the more pressure there is there are, the more pressure there is on the ones at the bottom. on the ones at the bottom.

The ones at the top feel very little The ones at the top feel very little pressure.pressure.

If you are standing at a high elevation, If you are standing at a high elevation, like on a mountain top, then there is less like on a mountain top, then there is less atmosphere above you than if you are atmosphere above you than if you are standing at the beach (sea level). standing at the beach (sea level).

Air Pressure on Other PlanetsAir Pressure on Other Planets

Venus Venus Same mass as the earth Same mass as the earth More gas molecules in its More gas molecules in its

atmosphere. atmosphere. Density = Density = massmass volumevolume At ground level, the At ground level, the

atmospheric pressure of atmospheric pressure of Venus is 90 atmospheres: Venus is 90 atmospheres: 90 ATM x 14.7 lbs/inch90 ATM x 14.7 lbs/inch2 2 = =

1323 lbs/inch1323 lbs/inch22..

Air DensityAir Density Because there is less air Because there is less air

pressure when we climb up pressure when we climb up mountains the air is less dense mountains the air is less dense at higher altitudesat higher altitudes

The molecules of air a farther The molecules of air a farther apart (some people describe this apart (some people describe this as “thin air”)as “thin air”)

We huff and puff, trying to get We huff and puff, trying to get enough oxygen into our lungs.enough oxygen into our lungs.

Our bodies are affected in two Our bodies are affected in two waysways1.1. Reduced air pressure decreases Reduced air pressure decreases

the amount of air we take in as we the amount of air we take in as we breathebreathe

2.2. There are fewer oxygen There are fewer oxygen molecules to breathe in each cmmolecules to breathe in each cm33

Measuring Air PressureMeasuring Air Pressure

Atmospheric pressure Atmospheric pressure is measured by an is measured by an instrument called a instrument called a barometerbarometer. Mercury – air pressing Mercury – air pressing

down on mercury force down on mercury force it up glass tubeit up glass tube

Aneroid – without Aneroid – without liquidliquid

Reading Barometric MeasurementsReading Barometric Measurements

A rising barometer indicates increasing air A rising barometer indicates increasing air pressurepressure Indicates good, dry, colder weatherIndicates good, dry, colder weather

A falling barometer indicates decreasing A falling barometer indicates decreasing air pressureair pressure Storms, rain and windy weatherStorms, rain and windy weather

Pressure UnitsPressure Units

Metric unit for air pressure is a Metric unit for air pressure is a millibarmillibar Average air pressure at sea level is 1013.25 Average air pressure at sea level is 1013.25

millibarsmillibars A line on a weather map connecting points A line on a weather map connecting points

of equal air pressure is called and of equal air pressure is called and isobarisobar

HomeworkHomework

Use the terms on page 50 of your Science Use the terms on page 50 of your Science Textbook to do the following:Textbook to do the following: Write a definition of each termWrite a definition of each term Write a sentence using each termWrite a sentence using each term Draw a picture of each term.Draw a picture of each term.

Read article on Air Pressure and complete Read article on Air Pressure and complete worksheet. worksheet.