D. Structures and Mechanisms · Simple Machines and Movement

Wedges and Screws

D2.3 D2.4 D2.5 A1.4 A1.1About 40 minutes

Learning goals

I can describe a wedge: a shape that is thick at one end and thin or pointed at the other, used to split, cut or hold things.
I can describe a screw: an inclined plane (a ramp) wrapped around a rod.
I can find wedges and screws in daily life, like knives, axes, doorstops, jar lids and light bulbs.
I can explain that a thin, sharp wedge needs less force to cut than a thick one, and follow safety rules with sharp tools.

Words to know

wedge
A simple machine that is thick at one end and thin or pointed at the other. It pushes things apart.

screw
A simple machine that is a ramp wrapped around a rod. You turn it to move it in or out.

thread
The spiral ridge that wraps around a screw.

split
To break something apart into pieces.

sharp
Having a very thin edge or point that cuts easily.

Two cousins of the ramp

In the last lesson you learned about the inclined plane, or ramp. Today you will meet two more simple machines that are really ramps in disguise!

A wedge is like two ramps back to back that you push into something.
A screw is a ramp wrapped around a rod.

The wedge

A wedge is thick at one end and thin or pointed at the other.

When you push the thin end into something, the slanted sides push the material apart. A wedge can:

split things (an axe splits firewood),
cut things (a knife cuts an apple),
hold things in place (a doorstop jammed under a door keeps it open).

The thinner and sharper the wedge, the less force you need to push it in. That is why a sharp knife cuts more easily than a dull one.

An axe blade stuck in the top of a log on a chopping block, with a crack splitting the log, and stacked firewood behind. — The axe blade is a wedge: thin at the edge and thick at the back. Its slanted sides push the wood apart and split the log. AI-generated image

Push down on the wedge. Its slanted sides push the wood apart, and the log splits.

Wedge	Its job
axe	splits firewood
knife	cuts food
doorstop	holds a door open
the point of a nail	pushes into wood
your front teeth	bite into an apple
zipper	small wedges inside the slider push the teeth apart or together

Wedges in daily life

The screw

A screw looks like a rod with a spiral ridge around it. The ridge is called the thread.

The thread is really a ramp wrapped around and around the rod! When you turn a screw, the thread pulls it into the wood a little bit at a time.

You turn it many times to move it a short distance. That makes it easier, and the thread holds very tightly. A screw holds things together better than a nail. Try pulling a screw straight out: you can't! You have to turn it back out.

A screw. The thread is a ramp that wraps around the rod.

Cut a paper triangle (a ramp). Roll it around a pencil, and the slanted edge makes a spiral, just like a screw's thread.

Screw	Its job
wood screw	holds pieces of wood together tightly
jar lid	twists on to seal the jar
bottle cap	twists on and off
light bulb base	twists into the lamp
spinning stool or desk chair	turns to go up or down
clamp or vise	squeezes things tightly while you work

Screws in daily life

Worked example

Let's test wedges: thin or thick?

We cut a block of playdough three ways. Which needs the smallest push?

Question: Does the shape of the wedge change how hard we have to push?
Predict: I think the thinnest edge will cut most easily.
Fair test: Same block of playdough, same size of cut. Change only the tool's edge.
Test 1: The edge of a plastic knife (thin). It slides in easily.
Test 2: The edge of a craft stick (medium). It takes a bigger push.
Test 3: The side of a thick, round marker (thick and blunt). It is hard to push in, and it squishes the playdough instead of cutting it.

The thinnest, sharpest wedge needed the smallest push. A thick, blunt shape needed the biggest push.

Worked example

Let's make a screw from a ramp

Can a ramp turn into a screw?

Cut a right triangle out of paper (a grown-up can help). The long slanted edge is a ramp.
Colour the slanted edge with a marker so it is easy to see.
Lay the pencil along the tall straight side of the triangle (not the slanted side). Roll the paper tightly around the pencil, toward the pointed end.
Look! The coloured edge makes a spiral around the pencil, like a screw's thread.

A screw's thread is an inclined plane wrapped around a rod.

Just for fun

Why do screws make such good friends?

Once they're in, they hold on tight and never let you down!

Safety with sharp tools

Wedges like knives, axes and saws are sharp. Only grown-ups use them. For our tests we use playdough and a plastic knife. Always cut away from your body, and keep fingers away from the edge.

Hands-on activity

Wedge and screw investigation

You need: playdough or a soft cheese slice, a plastic knife, a craft stick, a thick marker, paper, scissors, a marker, a pencil, and some things with screws (jar lids, bottle caps, a toy with screws).

Wedge test: Cut the playdough with the plastic knife edge, then the craft stick edge, then the side of the thick marker. Rank them from easiest to hardest.
Paper screw: Make the paper triangle and roll it on the pencil to see the spiral thread.
Screw hunt: Find three screws at home. Count how many turns it takes to take a jar lid off. Look at the thread inside the lid.
Research: Ask a grown-up to show you a wood screw and a nail. Feel the difference (gently). Which one has a thread?
Draw one wedge and one screw you found, and label what job each does.

What to look for: the thin knife edge is easiest; jar lids usually come off in one or two turns. Safety: a grown-up handles real screws, nails and anything sharp.

Your turn

What shape is a wedge?

Name two wedges.

What is a screw's thread?

Which simple machine is a jar lid?

Which cuts an apple more easily: a sharp knife or a dull knife?

What job does a doorstop do?

Who should use real knives and axes?

More practice: warm-up

Is an axe a wedge or a pulley?

Is a light bulb base a screw?

What do you do to a screw to make it go in: push it or turn it?

Which teeth are wedges: your front teeth or your back teeth?

What does a wedge do to the wood when it splits a log?

More practice: on your own

Why do we say a screw is a kind of inclined plane?

Why does a carpenter use screws instead of nails to hold a shelf tightly?

In the playdough test, which tool needed the biggest push, and why?

How is a wedge different from a ramp?

A spinning stool goes up when you turn the seat. Which simple machine is inside?

Challenge

Scissors have two simple machines. Name them and say what each part does.

Why does turning a screw many times make it easier to put into wood than hammering a nail?

A zipper is stuck open. Explain how the slider uses wedges to close it.

Check your understanding

Quiz

Which is a wedge?

A screw is really...

Which wedge needs the least force to cut?

Which is a screw?

What does a doorstop do?

Who should use a real axe?

For parents and teachers

Big idea: A wedge (thick at one end, thin at the other) splits, cuts or holds things. A screw is an inclined plane wrapped around a rod (D2.3, D2.4). The playdough test compares, qualitatively, how much force differently shaped wedges need (D2.5). Safety with sharp tools is a key Strand A skill (A1.4), and the screw hunt includes simple research (A1.1).

How to help: Point out wedges while cooking (with a grown-up holding the knife) and screws when opening jars. Make the paper screw together; seeing the spiral appear is a great "aha" moment.

Common difficulties:

Mixing up wedge and inclined plane. A ramp stays still; a wedge is pushed into something.
Thinking a screw is a nail. Look closely: a screw has a thread, a nail is smooth.
Safety: Keep real knives, axes, saws and loose screws away from young children. Use playdough and plastic knives for testing.