Air Pressure

What is mass?

• Mass is the amount of matter in an object.

• Matter refers to atoms and molecules

• Even if gravity changes, mass stays the same

• Mass is not the same as weight. Weight depends on gravity. Weight is measured in pounds or kilograms, ounces or grams.

What is gravity?

A force of attraction between any objects that have mass• On Earth, the center of

gravity is Earth’s inner core.• Earth’s gravity keeps the

atmosphere from floating away, makes things fall, and prevents us from jumping too high

• Anything with mass has a gravitational pull.

• More massive = more gravitational pull

What is Density?• Density is a way of describing

how tightly packed a substance is, or how close together its molecules and atoms are, or how much mass is in a given volume

• More mass in a given volume

= more dense• Less mass in a given volume

= less dense• Dense things cram a lot of

mass into a small space.

What is meant by VOLUME?• In math and science Volume is usually defined as the amount of space that something takes up.

larger force = larger pressure

smaller force = smaller pressure

larger area = smaller pressure

smaller area = larger pressure

Pressure is force pushing on an area.

pressure = force area

What is Pressure?

Air Pressure = molecules pushing (exerting force) on other molecules

• At sea level a 300-mile high column of air molecules push down with a force of 14.7 pounds per square inch (psi), or 1 kg per

square centimeter (kg/cm2).• That’s like having an elephant

sitting on your head.

We aren't pushed around or squashed by this pressure

because the forces on all sides of us balance one another out.

Air pressure is measured with a barometer .

Air pressure is measured in millibars or “inches of mercury”

29.92” Hg = 1,013 mbAverage height of barometric column at sea level

Altitude affects air pressure

pressure = force area

Increasing altitude means less and less air molecules per given area. Less molecules in a given area means lower density. Less molecules also means less pressure (force

exerted) because fewer molecules collide. Also, it’s colder up there which causes molecules to

slow down and collide with less force.

When amount of mass in a given volume decreases, the

density also decreases.

Altitude increases and amount of air molecules decrease.

Less air molecules per given area also mean atmosphere is less DENSE at higher altitudes.

altitude density

Areas of high pressure push towards areas of low pressure.

Temperature affects pressure

• Heating causes molecules to move faster.• Moving faster increases the amount of

collisions between individual molecules and the relative force of those collisions.

• More force results in more pressure.• Faster moving molecules exert greater

pressure on a rigid container. If there is no container, then faster molecules will move apart.

What happened?• Burning candle consumes oxygen

from air inside glass.• This reduces mass & density of

air in glass and therefore, the air pressure.

• Candle goes out, heat source is gone causing gas molecules to slow down further drop in air pressure.

• Outside pressure is now greater than inside.

Difference in air pressure pushes water from pan (outside) up into the glass.

High pressure area pushes towards low pressure area.

Click the links below for more fun!

Basketball and Air Pressure Interactive

NOVA Floating and Sinking Interactive

Air Pressure Video Clip

Eureka! Volume and Density

Eureka! Buoyancy