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You’ve heard the word pressure being used in relation to blood pressure or the weather (high- and low-pressure weather systems).

Pressure  is defined as how much force is on a certain area.

     Pressure = Force / Area      P = F / A

There are different units for measuring pressure.

Units of pressure


One atmosphere = the weight of all the air in the Earth’s atmosphere, weighing down over just one square inch of the Earth’s surface.


The standard unit for pressure is the Pascal, in the metric system.

1 Pascal = 1 Newton of force distributed over 1 square meter.

1 Pa = 1 N/m^2

Pascal unit of pressure

Image from PPT Power Of Hydraulic. Linnaeus University, Javad Mohammadi Majd

Bar and millibar

These units are often used in meterology.

1 bar = 100,000 Pascals

This is slightly less than the average atmospheric pressure on Earth at sea level.


Pounds/square inch

We often use this when measuring tire pressure.

Pressure inside a car tire

Image from College Physics, OpenStax, Rice University

Millimeters of mercury

Okay, this unit sounds odd. Mercury is a metal that is liquid at room temperature.

Liquid mercury metal

First, let’s simplify by using common abbreviations. “millimeters of mercury” will now be written as mm Hg.

And why mention “millimeters?” A millimeter is just a tiny length. Liquid drops of mercury obviously have no special length, they just act like any other liquid, filling volumes.

So what does this mm Hg mean? It comes from the original way that scientists first measure air pressure.

In 1644 Evangelista Torricelli put mercury in a glass tube. When the air pressure increased, the mercury in the glass tube moved one way, and when the air pressure decreased, the mercury moved the other way.

Torricelli mercury barometer pic by Camille Flammarion

Torricelli inventant le baromètre à mercure, gravure figurant dans les livres de Camille Flammarion

Torricelli used a ruler marked in inches (although in later years, people began marking it in millimeters.) The unit of mm HG is still used when measuring blood pressure.

Measure blood pressure

How does force work?

Use your finger to put a force on your skin. There might be a lot of force in your finger, but it is spread out over a wide-ish area, over 1 square centimeter. So the force per area, pressure, is small.

But now put exactly the same amount of force over a very tiny area. Now the force per area, pressure, is large enough to puncture the skin.

Same force smaller area more pressure needle

Image from College Physics, OpenStax, Rice University


Gas pressure is exerted perpendicular to any surface. Here is the interior of a tire.

Pressure inside a car tire

Image from College Physics, OpenStax, Rice University


Swimmers feel pressure from the water on all sides.

Hmm, note anything funny about the length of the red force lines here? The longer the line, the more that the force is. Are the forces on the swimmer balanced or not? Think about the concept of buoyancy!

Pressure exerted on all sides of a swimmer and buoyant force (buoyancy)


Online labs

PhET gas properties

Level 1: Memorization of vocabulary; ability to answer true/false questions; matching questions
Level 2: Show comprehension of physics concepts, at the Mass Learning Standards level, by running PhET simulations of gas, pressure and volume, and then explaining the observed phenomenon in complete sentences, at grade level grammatical accuracy.

Where does air pressure actually come from?

How does air pressure change as a function of altitude?

Why aren’t we crushed by the air pressure around us? That pressure is pointing inwards, on every point of our body.

Because we have an equal amount of pressure inside our bodies!
Fluids and gases inside our body cells have their own pressure.
Internal pressure outward = air pressure inward

What would happen if we removed this internal pressure inside us, or inside any object? Let’s take a large object and evacuate it: As the level of gas pressure inside the object decreases, what occurs?

Air pressure demonstration imploding tanker train

Atmospheric pressure versus altitude

Changes in atmospheric pressure with height.

Atmospheric pressure is the weight of the air above.

At sea level, average air pressure =

1000 millibars = 1 kilogram / cm^2 = 14.7 pounds / in^2

One half of the atmosphere lies below an altitude of 5.6 kilometers.

Above 100 km = 0.00003 percent of the atmosphere


chap 17

Graph reading skills

We have 2 different Y-axes:
Both are for altitude, but one is in km while the other is in miles.

We have 2 different X-axes:
Both are for temperature, but one is in degrees C while the other is in degrees F.

At what altitude do air molecules have the warmest temperature?

Higher level question: Why would it nonetheless feel cold if you were in this warmer region of the atmosphere?

Thermal structure of the atmosphere

figure 6 Tarbuck Lutgens

Troposphere – bottom layer – temperature decreases with an increase in altitude. This is where our weather occurs.

temperature drops, to a height of about 12 kilometers

Stratosphere – here temperature remains constant to about 20 kilometers. Temp then gradually increases until the stratopause, at 50 km

Temp increase here because ozone is concentrated here: ozone absorbs ultraviolet radiation from the sun.

Mesosphere – temp decreases with altitude, until 80 km. Air temperatures approach −90°C.

Thermosphere – contains only a tiny fraction of the atmosphere’s mass.

Temp increases here because O2 and N2 gas molecules absorb short-wave, high-energy solar radiation.

Yet it would feel cold if you exposed yourself to these hot air molecules! Why?

Because although the air molecules are vibrating faster (‘hotter’) there are far fewer of them.

What would keep you warmer? Having only one huge fireplace, for a very large apartment building? Or having small fireplaces in every single apartment?

Heat = ( temp of air molecules ) x ( # air molecules )

Gas laws

At constant temperature, the product of a gas’s pressure and volume is constant.

Boyle's law pressure temp

Learning Standards

2016 Massachusetts Science and Technology/Engineering
HS-PS1-3. Cite evidence to relate physical properties of substances at the bulk scale to spatial arrangements, movement, and strength of electrostatic forces among ions, small molecules, or regions of large molecules in the substances. Make arguments to account for how compositional and structural differences in molecules result in different types of intermolecular or intramolecular interactions.

HS-PS2-8(MA). Use kinetic molecular theory to compare the strengths of electrostatic forces and the prevalence of interactions that occur between molecules in solids, liquids, and gases. Use the combined gas law to determine changes in pressure, volume, and temperature in gases.

A Framework for K-12 Science Education: Practices, Crosscutting Concepts, and Core Ideas (2012)

PS1.A Structure of matter (includes PS1.C, nuclear processes)

That matter is composed of atoms and molecules can be used to explain the properties of substances, diversity of materials, how mixtures will interact, states of matter, phase changes, and conservation of matter. States of matter can be modeled in terms of spatial arrangement, movement, and strength of interactions between particles. Characteristic physical properties unique to each substance can be used to identify the substance.


The arrangement and motion of atoms vary in characteristic ways, depending on the substance and its current state (e.g., solid, liquid). Chemical composition, temperature, and pressure affect such arrangements and motions of atoms, as well as the ways in which they interact. Under a given set of conditions, the state and some properties (e.g., density, elasticity, viscosity) are the same for different bulk quantities of a substance, whereas other properties (e.g., volume, mass) provide measures of the size of the sample at hand.

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