D. Structures and Mechanisms · Simple Machines and Movement

Wheel and Axle: Rolling Makes It Easier

D2.3 D2.4 D2.5 A1.2 A1.5About 35 minutes

Learning goals

I can describe a wheel and axle: a wheel that turns together with a rod through its middle.
I can find wheels and axles in daily life, like wagons, bikes, doorknobs and rolling pins.
I can measure and compare the force needed to drag something and to roll it on wheels.

Words to know

wheel
A round part that turns.

axle
The rod through the middle of a wheel. The wheel turns on it or with it.

wheel and axle
A simple machine made of a wheel joined to a rod.

friction
A force that slows things down when two surfaces rub together.

gear
A wheel with teeth around the edge. Gears fit together to turn each other.

Drag it or roll it?

You need to move a big bag of groceries down the street. You could drag it along the sidewalk. Or you could put it in a wagon and roll it.

Which is easier? The wagon, of course! The wagon has wheels and axles, one of the six simple machines.

A red wagon on a sidewalk carrying two paper bags full of vegetables and bread. — Each wheel turns on a metal rod called an axle. Rolling the groceries in the wagon is much easier than dragging the bags. AI-generated image

What is a wheel and axle?

A wheel and axle has two parts:

a round wheel,
a rod through its middle called the axle.

The wheel and axle turn together, or the wheel turns around the axle.

How does it help?

1. Rolling is easier than dragging. When you drag something, it rubs on the ground. This rubbing makes a force called friction that slows it down. A wheel rolls, so there is much less rubbing. Rolling takes a smaller pull.

2. A big wheel turns a small axle with more strength. Turning a doorknob is easy. But if the knob fell off, turning the thin rod inside with your fingers would be really hard! The big wheel (the knob) makes it easier to turn the axle.

A wheel and axle: two wheels joined by a rod called the axle, like on a toy car or a wagon.

🚲🛒🛹🚗🦽

Bike, shopping cart, skateboard, car and wheelchair all roll on wheels and axles.

Wheel and axle	How it makes work easier
wagon or wheelbarrow	rolls heavy things instead of dragging them
doorknob	the big knob turns the small rod that opens the latch
steering wheel	a big wheel makes it easier to turn the car
rolling pin	rolls smoothly over dough while you hold the handles
pencil sharpener with a crank	the handle goes around in a big circle to turn the blades
wheelchair	rolls a person along with less effort

Wheels and axles in daily life

Gears are wheels too

A gear is a wheel with teeth on its edge. When one gear turns, its teeth push the next gear and make it turn. A bicycle has gears joined by a chain. A hand-crank egg beater has gears too.

Worked example

Let's measure: drag or roll?

We put 4 blocks in a toy dump truck. We pull it with a rubber band tied to the front and measure how far the rubber band stretches. A bigger stretch means a bigger pull.

Question: Is it easier to pull the truck on its wheels, or to drag it with no wheels touching?
Predict: I think rolling on its wheels will need a smaller pull.
Fair test: Same truck, same 4 blocks, same rubber band, same floor. Change only wheels or no wheels.
Test 1: Turn the truck upside down so the wheels are in the air. Pull it slowly. The rubber band stretches 6 cm.
Test 2: Turn the truck right side up, on its wheels. Pull it slowly. The rubber band stretches only 1 cm.
Compare: 6 cm is more than 1 cm. Dragging needed a much bigger pull.

Rolling on wheels needed a much smaller pull than dragging. The wheels and axles made the job easier. (Your numbers may be different. That's okay!)

Test	Rubber band stretch	Easy or hard?
dragged upside down (no wheels)	6 cm	hard
rolled on its wheels	1 cm	easy

Our results

Worked example

Let's explain a doorknob

Why does a door have a round knob instead of just a small metal rod sticking out?

The knob is the wheel. The rod inside the door is the axle.
When you turn the big knob, the axle turns with it and pulls back the latch.
Your hand moves around a big circle on the knob, while the thin axle only turns in a tiny circle. So your small twist turns the axle with more strength.
So a small twist on the big knob does the job easily.

The big knob is a wheel that makes it easier to turn the small axle.

Your hand goes around the big wheel (the knob) while the small axle turns only a tiny circle. That turns the axle with more strength.

Hands-on activity

Rubber band pulling test

You need: a toy truck or car that can carry a few small things, some blocks or coins for a load, a rubber band, string, and a ruler.

Tie the string to the front of the truck and loop the rubber band onto the string. Hold the rubber band to pull.
Put the load in the truck. Lay the ruler beside the rubber band.
Turn the truck upside down. Pull it slowly and steadily across the floor. How long does the rubber band stretch? Write it down.
Turn the truck right side up and do it again the same way.
Make a chart like ours. Which needed a bigger pull?
Share: Tell a grown-up your results using the words wheel, axle and friction.

What to look for: the rubber band stretches more when dragging. Safety: don't stretch the rubber band too far, and keep it away from faces.

Just for fun

Why does everybody want to bring the wagon to the picnic?

Because it's always on a roll!

Your turn

What are the two parts of a wheel and axle?

Is it easier to drag a heavy box or roll it in a wagon?

Why is rolling easier than dragging?

Name three things with wheels and axles.

Which part of a doorknob is the wheel?

What is a gear?

In the truck test, a bigger stretch of the rubber band means...

More practice: warm-up

Is a rolling pin a wheel and axle?

What is the rod in the middle of a wheel called?

Name the simple machine on a skateboard.

Do bike wheels move round and round or back and forth?

Which is easier to push: a full shopping cart on its wheels, or a full box on the floor?

More practice: on your own

A steering wheel is big. Why does a big wheel help the driver?

Sam drags a box: the rubber band stretches 8 cm. On wheels: 2 cm. How many centimetres less did rolling need?

Why did we use the same truck and the same load in both tests?

A wheelchair user rolls along the sidewalk. How do the wheels and axles help?

Why do suitcases often have wheels?

Challenge

A doorknob falls off. Why is it now hard to open the door?

Long ago, people moved heavy things on logs laid on the ground. Why did that help? How is a wagon even better?

Design a way to bring three heavy bags of soil from the car to the garden. Use a wheel and axle and explain how it helps.

Check your understanding

Quiz

The rod through the middle of a wheel is the...

Why is it easier to roll a box in a wagon than to drag it?

Which is a wheel and axle?

A gear is...

Dragging stretched the rubber band 7 cm. Rolling stretched it 2 cm. Which needed less force?

Which one does not use a wheel and axle?

For parents and teachers

Big idea: A wheel and axle is a wheel joined to a rod. Rolling reduces friction, so moving loads on wheels takes less force, and a big wheel turns a small axle more easily (D2.3, D2.4). The rubber band test lets your child measure and compare forces in centimetres (D2.5), as a fair test (A1.2), and share results in a chart (A1.5).

How to help: Count wheels on a walk (cars, strollers, bikes, buses). Try opening a door by gripping only the narrow part of the handle near the door, then the end of the handle. Feel the difference.

Common difficulties:

Thinking the load gets lighter on wheels. The weight is the same; the pull needed is smaller because there is less friction.
Forgetting the axle. Point to the rod in the middle of a toy car's wheels.
Measuring unevenly. Pull slowly and steadily, and read the stretch while the truck is moving.