D. Structures and Mechanisms · Forces Acting on Structures

Internal Forces: Compression, Tension, Torsion and Shear

D2.1 A1.2 A1.5About 40 minutes

Learning goals

I can explain what an internal force is.
I can identify four internal forces: compression, tension, torsion and shear.
I can describe the effect each internal force can have on a structure.
I can explain that a bending beam is squeezed on one side and stretched on the other.

Words to know

structure
Something built (or found in nature) that supports a load and holds its shape, like a bridge, a chair or a tree.

internal force
A force acting inside the parts of a structure, in the materials it is made of.

compression
An internal force that squeezes or pushes the parts of a material together. It tends to make a part shorter.

tension
An internal force that stretches or pulls the parts of a material apart. It tends to make a part longer.

torsion
An internal force that twists a material, because its two ends are turned in opposite directions.

shear
An internal force that makes one part of a material slide past the part next to it, because two forces push in opposite directions close together.

buckle
To bend or fold sideways suddenly when squeezed too hard, like a long thin straw.

Twist, squeeze, snap!

Wring out a wet facecloth. You twist one end one way and the other end the other way. Now press on the top of a stack of playing cards and slide your hand sideways. The top cards slide over the bottom cards.

In both cases, forces are acting inside the material. Engineers call these internal forces. Today you will learn their names and what they can do to a structure.

Remember from Grade 3

In Grade 3 you learned that a structure supports a load. You met two forces inside structures: compression (a squeeze) and tension (a stretch). You found struts, which are squeezed, and ties, which are stretched.

In Grade 5 you will add two more internal forces, torsion and shear, and look closely at what each force can do to a structure.

A steel truss bridge made of many triangles crossing a calm river. — In a truss bridge, some steel bars are squeezed (compression) and others are stretched (tension). The triangles help the bridge keep its shape. AI-generated image

Outside forces cause inside forces

When something outside pushes or pulls on a structure, like snow on a roof or a person sitting on a chair, every part of the structure has to push or pull back. The forces inside the parts are internal forces.

There are four internal forces to know:

Compression squeezes a material. The part gets a tiny bit shorter.
Tension stretches a material. The part gets a tiny bit longer.
Torsion twists a material. One end turns one way while the other end turns the other way.
Shear pushes one part of a material one way and the part right beside it the other way, so the parts try to slide past each other.

The four internal forces. The arrows show which way the forces push or pull. In torsion, the two ends are turned in opposite directions. In shear, the top part is pushed one way and the bottom part the other way.

What each force can do

A well-built structure pushes and pulls back without changing much. But if an internal force is too big for the material, the structure can be damaged:

Too much compression can crush a part, or make a long, thin part buckle (bend out sideways all of a sudden). Stand a drinking straw on the table and press down on top. It buckles.
Too much tension can stretch a part out of shape or make it snap, like an elastic band pulled too far.
Too much torsion can twist a part out of shape or make it break, like a twist-tie twisted again and again.
Too much shear can cut or tear a part, or make a bolt or nail break where two pieces slide past each other. Scissors cut paper using shear. That is why big scissors are called shears!

Internal force	Everyday example	What too much can do
compression	the legs of a chair when you sit on it	crush or buckle
tension	the chains of a swing when you swing	stretch or snap
torsion	a key when you turn it in a lock	twist out of shape or break
shear	paper being cut by scissors	cut, tear or break across

Internal forces in everyday things

Bending: two forces at once

When a load pushes down on the middle of a beam (a long flat part held up at both ends), the beam bends. Look at the picture:

The top of the beam is squeezed. It feels compression.
The bottom of the beam is stretched. It feels tension.

So bending is really compression and tension working together. A beam can break if its top gets crushed or its bottom gets pulled apart. A beam that bends too much sags.

A beam held up at both ends bends under a load in the middle. The top is squeezed (compression) and the bottom is stretched (tension). The bend is drawn bigger than in real life so you can see it.

How to tell them apart

Ask what the force is trying to do to the part:

make it shorter? compression
make it longer? tension
twist it? torsion
make two pieces slide past each other? shear

Worked example

Let's find the internal forces in a swing set

A child is swinging on a playground swing set. Find the internal forces in the chains, the legs and the top bar.

Chains: the child's weight pulls down on them. They are being stretched, so they feel tension.
Legs: the top bar and everything hanging from it press down on the legs, and the ground pushes up. The legs are squeezed, so they feel compression.
Top bar: the swings hang from the middle, and the bar is held up at its ends, so it bends a tiny bit. Its top feels compression and its bottom feels tension.
Bolts: where the chain hooks hold onto the bar, the load pulls one way and the bar holds the other way, so the bolts feel shear.

Chains: tension. Legs: compression. Top bar: bending (compression on top, tension on the bottom). Bolts: shear.

Worked example

Let's be a structure detective

Name the internal force that most likely caused each kind of damage: (a) a tall, thin metal pole holding up a heavy sign has bent out sideways in the middle; (b) a sign on one pole was spun by wind and its pole is now twisted; (c) a rope holding a boat to the dock has snapped; (d) a bolt joining two metal plates was cut straight across where the plates meet.

(a) The heavy sign pushes down on a long thin pole. It was squeezed and bent out sideways: that is buckling, caused by compression.
(b) The wind turned the sign one way while the ground held the bottom of the pole: torsion.
(c) The boat pulled on the rope until it broke: tension.
(d) The plates were pushed in opposite directions and the bolt was cut where they slide past each other: shear.

(a) compression (buckling), (b) torsion, (c) tension, (d) shear.

Just for fun

Riddle: Why did the elastic band go to see the doctor?

It was feeling way too much tension!

Hands-on activity

Sponge model of internal forces

You need: a clean, new rectangular kitchen sponge (the soft kind, with no scrubby side if you can), a marker and a ruler.

Draw 5 straight lines across the sponge, about 2 cm apart, like the rungs of a ladder. Draw one long line down the middle the long way.
Compression: hold the sponge by its two short ends and push them toward each other. What happens to the lines? (They get closer together.)
Tension: pull the two ends apart gently. (The lines get farther apart.)
Torsion: twist one end one way and the other end the other way. (The long line becomes a spiral.)
Shear: lay the sponge flat. Press one hand on the top and slide it forward while your other hand holds the bottom still. (The lines tilt into slants.)
Bending: hold both ends and push the middle down with a thumb. Look at the top edge and the bottom edge. (The top lines squeeze together. The bottom lines spread apart.)
Make a chart with a sketch of the sponge for each force. Label it with the force's name.

What to look for: the sketches match the force: closer lines for compression, spread lines for tension, a spiral for torsion, slanted lines for shear, and both closer and spread lines for bending. Scientists use models like this to see things that are too small to see in real structures.

Your turn

What is an internal force?

Which internal force squeezes a material?

Which internal force twists a material?

Which internal force makes two parts try to slide past each other?

A rope in a game of tug-of-war feels which internal force?

What can happen to a long, thin part if it feels too much compression?

When a beam bends under a load in the middle, which side is squeezed?

More practice: warm-up

Which internal force stretches a material?

Scissors cut paper using which internal force?

Which internal force do you use when you wring out a wet towel?

The legs of a table feel which internal force?

True or false: internal forces happen inside structures, even when they stand still.

More practice: on your own

When you turn a doorknob, which internal force acts in the shaft of the knob? Why?

A hammock is tied between two trees. Name the force in the ropes and the force in the trees' trunks near the ground. Explain.

Why does a long, thin uncooked spaghetti noodle buckle more easily than a short one when you push on its ends?

A hole punch makes a hole in paper. Which internal force does it use? Explain.

A shelf sags in the middle under heavy books. Describe the internal forces in the shelf.

Challenge

Concrete is very strong when squeezed but cracks easily when stretched. A concrete beam holds a load in the middle. On which side will cracks most likely start? Explain.

You use a screwdriver to turn a very tight screw. Which internal force acts in the metal shaft of the screwdriver? What could happen if the screw is far too tight?

Ravi says, "A chair that nobody is sitting on has no internal forces." Is he right? Explain using what you learned.

Check your understanding

Quiz

Which internal force twists a material?

The cables that hold up a suspension bridge feel which internal force?

What can happen to a tall, thin column if it feels too much compression?

Scissors cut paper using which internal force?

A beam is held up at both ends and has a load in the middle. Which is true?

Which is an example of compression?

For parents and teachers

Big idea (D2.1): Grade 5 students identify internal forces acting on a structure (compression, tension, torsion and shear) and describe their effects. Your child met compression and tension in Grade 3; torsion and shear are new. Bending is a combination of compression (top) and tension (bottom) in a beam held at both ends.

How to help: Go on a "force hunt" around the house. Point to a part (a towel bar, a door hinge, a clothesline, the leg of a table) and ask, "Is it being squeezed, stretched, twisted, or are two parts trying to slide past each other?" Ask your child to explain how they know (A1.5, communicating with science vocabulary).

Common difficulties:

Mixing up shear and tension. Tension pulls a part apart along its length; shear pushes two parts sideways past each other. Scissors and a stack of sliding cards show shear well.
Thinking a still structure has no forces. Remind your child that gravity always acts, so every loaded part is pushing or pulling back.
Safety: use a clean new sponge for the activity, not one from the kitchen sink. Do not stand on cans or other objects to test crushing.