D. Structures and Mechanisms · Flight

Air Is Real Stuff: Properties of Air and How Flight Uses Them

D2.1 A1.2 A1.4About 45 minutes

Learning goals

I can describe six properties of air.
I can identify ways that flying machines and people who fly use the properties of air.
I can explain how a hot-air balloon rises.
I can carry out simple air experiments safely and explain what I observed.

Words to know

air
A mixture of gases, mostly nitrogen and oxygen, that surrounds the Earth.

property
A feature that describes what something is like or how it behaves.

air pressure
The push of air on everything it touches. Air pushes in all directions.

compress
To squeeze something into a smaller space. Air can be compressed.

expand
To spread out and take up more space. Air expands when it is heated.

air resistance
The push of air against something moving through it. It slows the object down. (Also called drag.)

buoyant force
An upward push from a liquid or gas on an object in it.

What holds a giant plane up?

A big passenger jet can have a mass of hundreds of tonnes. That's heavier than a hundred cars! Yet it zooms through the sky, held up by something you can't even see: air.

To understand flight, you first need to understand air. Today you will find out what air is like (its properties) and how people use those properties to fly.

A large white passenger jet seen from below, flying in a clear blue sky. — This jet has a mass of many tonnes, yet invisible air holds it up. Flight depends on the properties of air. AI-generated image

Remember from Grade 2 and Grade 5

In Grade 2 you learned that air is all around us, that air takes up space, that air has mass, and that moving air (wind) can push things.

In Grade 5 you learned that air is a gas. Its particles are far apart and moving all the time, so a gas can be compressed (squeezed).

In Grade 6 you will put all of this together and see how each property helps things fly.

Six properties of air

Air takes up space. An "empty" bottle is really full of air.
Air has mass. A ball pumped full of air has more mass than the same ball when it is flat. Because air has mass, gravity pulls on it, so air has weight too.
Air can be compressed. You can squeeze more air into a space, like when you pump up a bike tire. Squeezed air pushes back hard.
Air expands when it is heated and contracts (takes up less space) when it is cooled. Warm air is lighter for its size than cool air, so warm air rises.
Air pushes on things (air pressure). The air all around you is pushing on everything, in all directions: up, down and sideways. Air pressure is lower high up, because there is less air above pushing down.
Moving air pushes on things, and air pushes back on things that move through it. Wind can push a kite or a sailboat. When you ride your bike fast, you feel the air pushing against you. That push is air resistance, also called drag.

Air can be compressed. The tip is sealed, so pushing the plunger in does not let any air out: the same particles are squeezed into a smaller space. That squeezed air pushes back hard on the plunger.

Property of air	How flight uses it
Air takes up space and has mass	Wings and propellers push on air. Because air is real stuff with mass, it pushes back on them.
Air can be compressed	Jet engines squeeze (compress) air before burning fuel in it. Plane tires hold air or another gas under pressure so they can carry a heavy plane.
Warm air expands and rises	A hot-air balloon rises because the heated air inside is lighter for its size than the cooler air outside. Hawks and vultures soar in rising columns of warm air.
Air pushes in all directions (air pressure)	Wings make lift because the air pushes harder on the bottom of the wing than on the top. An altimeter measures air pressure to tell pilots how high they are. Big planes are pressurized (air is pumped in) because air high up is too thin to breathe easily.
Moving air pushes; air resists motion	Kites are pushed up by the wind. Parachutes use air resistance to slow a fall. Planes take off into the wind to get more air rushing over their wings. Airports use wind socks to show wind direction.

Properties of air and how flight uses them

A hot-air balloon. The burner heats the air inside, so the air expands and becomes lighter for its size than the cooler air around it. The cooler air pushes the balloon up (the buoyant force). Gravity pulls the balloon, basket and people down (weight). When the push up is bigger than the weight, the balloon rises.

Hot-air balloons: warm air rises

Remember buoyancy from Grade 3? Water pushes up on a boat. Air can push up too! A hot-air balloon uses a burner to heat the air inside it. The heated air expands, and some of it spills out of the open bottom. Now the balloon holds less air than it would if the air were cool, so it is lighter than the same amount of space filled with cooler outside air.

The cooler air around the balloon pushes it up. When that push is bigger than the balloon's weight, up it goes! To come down, the pilot lets the air cool (or lets some hot air out through a vent at the top).

Big idea

Air is invisible, but it is matter: it takes up space, has mass and pushes on things. Every flying thing, from a bee to a jumbo jet, works by pushing on air and letting air push back.

Worked example

Let's match properties to flight

Name the property of air that each one uses: (a) a parachute slows a skydiver; (b) a hot-air balloon rises; (c) a jet engine squeezes air before it burns fuel; (d) a pilot reads an altimeter to know how high the plane is.

(a) The parachute catches a lot of air as it falls. The air pushes back on things moving through it. That push is air resistance (drag).
(b) The burner heats the air inside the balloon. Warm air expands and is lighter for its size, so the cooler air around pushes the balloon up.
(c) Squeezing air into a smaller space uses the fact that air can be compressed.
(d) The altimeter measures air pressure. Air pressure is lower high up, so lower pressure means the plane is higher.

(a) air resistance, (b) warm air expands and rises, (c) air can be compressed, (d) air pressure is lower higher up.

Worked example

Let's explain an observation

Lina has a bag of chips from the store. She takes it on a plane. High in the air, the bag puffs up and looks ready to pop. Why?

Inside the bag there is some air, sealed in at the factory.
Up high, the air pressure in the cabin is lower than on the ground (planes are pressurized, but not all the way to ground level).
With less air pushing on the outside of the bag, the air inside pushes the bag outward and it expands.

The air inside the bag pushes out harder than the lower-pressure air outside pushes in, so the bag puffs up.

Just for fun

Riddle: What fills a whole room but takes up no space?

Light! (Not air. As you just learned, air really does take up space.)

Hands-on activity

Air experiments at the kitchen table

You need: a drinking glass, a large bowl of water, an empty plastic bottle, a balloon, a bowl of cold water, a bowl of hot tap water (not boiling), three sheets of paper and, if you have one, a clean plastic medicine syringe (no needle).

Safety: ask a grown-up to run the hot tap water for you. Use hot tap water only, never boiling water. Balloons are a choking hazard: keep balloons and balloon pieces away from little children, and throw away any balloon that pops.

Air takes up space: crumple the paper and push it into the bottom of an empty glass. Turn the glass upside down and push it straight down into a bowl of water. Lift it straight out. Is the paper dry? (It should be, because the air in the glass kept the water out.)
Air can be compressed: pull the syringe plunger out, put your finger tightly over the tip, and push the plunger. You can push it in partway, then it pushes back. Let go: the plunger springs back. (Compressed air pushes back.)
Air expands when heated: stretch the balloon's opening over the mouth of the empty bottle. Stand the bottle in the bowl of hot water for 2 minutes. Then move it to the cold water. What happens to the balloon each time?
Air resistance: hold two sheets of paper high, one flat and one crumpled into a ball. Drop them at the same time. Which lands first?

Record: for each test write a prediction, what you observed, and which property of air it shows.

What to look for: (1) the paper stays dry: air takes up space; (2) the plunger stops and springs back: air can be compressed; (3) the balloon puffs up in hot water and droops in cold water: air expands when heated and contracts when cooled; (4) the ball lands first because the flat sheet has much more air resistance.

Your turn

Name three properties of air.

Why does a hot-air balloon rise?

What does a parachute use to slow a fall?

Is air pressure higher or lower high in the sky? Why?

Why do big airplanes have pressurized cabins?

Which property of air lets you pump up a bike tire?

Why do planes usually take off into the wind?

More practice: warm-up

True or false: an "empty" bottle is full of air.

True or false: air has no mass.

What happens to air when it is heated?

What do we call the push of air against something moving through it?

What tool tells a pilot how high the plane is by measuring air pressure?

More practice: on your own

A ball is weighed when flat, then pumped up and weighed again. What will the scale show? Which property of air does this prove?

You put a balloon over the mouth of a bottle and put the bottle in the freezer. Predict what happens and explain.

Hawks often circle higher and higher without flapping on sunny days. Which property of air are they using?

Why does a crumpled ball of paper fall faster than a flat sheet of the same paper?

Your ears may "pop" when a plane climbs. Use air pressure to explain why.

Challenge

A hot-air balloon pilot wants to come down slowly. What could the pilot do? Explain using properties of air.

Hot-air balloons usually fly in the cool early morning or evening, not on a hot afternoon. Suggest one science reason.

Design a test to show that air takes up space, using a glass, a bowl of water and a tissue. Write the steps and what you expect.

Check your understanding

Quiz

Which is a property of air?

Why does a hot-air balloon rise?

A parachute works mostly because of:

High in the sky, air pressure is:

A jet engine squeezes air before burning fuel. Which property does this use?

Which tool uses air pressure to measure how high a plane is flying?

For parents and teachers

Big idea (D2.1): Grade 6 students identify flight-related applications of the properties of air: air takes up space, has mass, can be compressed, expands when heated, exerts pressure in all directions (lower pressure higher up) and resists motion. Your child met several of these in Grade 2 and Grade 5; the new part is linking each property to flight.

How to help: Do the activity together and ask your child to say each result in a full sentence: "The paper stayed dry, which shows that air takes up space." Ask for a prediction before each test (A1.2, scientific experimentation).

Common difficulties:

"Hot air rises because heat is light." It is the air that rises: heated air expands, so it is less dense than the cooler air around it, and the cooler air pushes it up.
Thinking air is "nothing". Air is matter with mass. A pumped-up ball has a slightly larger mass than a flat one.
Safety (A1.4): a grown-up handles the hot water (tap water only, never boiling). Balloons and broken balloon pieces are a choking hazard for young children. Use a clean syringe with no needle.