Grade by Grade
D. Structures and Mechanisms · Forces Acting on Structures

Nature's Engineers: How Plants and Animals Withstand Forces

D2.4 A1.2 A1.5 A3.2About 40 minutes
Learning goals
  • I can describe physical characteristics that help animals protect themselves from harmful forces.
  • I can describe physical characteristics that help plants withstand forces like wind, snow and waves.
  • I can connect these characteristics to structural ideas like domes, hollow tubes and flexibility.
  • I can test how a hollow tube compares with a flat strip.

Words to know

physical characteristic
A feature of a living thing's body, like a shell, a thick skull, deep roots or a bendy stem.
shell
A hard outer covering that protects a soft body, like a turtle's or a clam's.
exoskeleton
A hard covering on the outside of an animal's body, like a beetle's.
dome
A rounded, curved shape like half a ball. It spreads a push over the whole shape.
flexible
Able to bend without breaking, and spring back.
anchor
Something that holds a thing firmly in place, like roots holding a tree in the ground.

Built by nature

A spruce tree stands through blizzards. A mussel clings to a rock while waves crash over it. A turtle pulls into its shell, and a hungry predator can't crack it.

Plants and animals are structures too! Their bodies have physical characteristics that protect them from harmful forces: crushing, wind, waves, heavy snow and hard hits. Many of the same ideas engineers use (domes, triangles, hollow tubes, flexibility and anchors) are found in nature.

Animals: shells, skulls and anchors

  • Turtles (like the painted turtle and the snapping turtle in Ontario) have a hard, domed shell. A dome spreads a push over the whole shape, so a bite or a squeeze doesn't land on one weak spot. It works like an arch in every direction.
  • Snails and clams have hard shells that protect their soft bodies from being crushed.
  • Beetles have a hard exoskeleton on the outside of their bodies.
  • Mussels attach to rocks with many tough threads. The threads hold on in tension, so crashing waves can't wash the mussel away.
  • Muskoxen have very thick skulls and wide, heavy horn bases that cover their foreheads. Males charge and clash heads in the mating season, and this protection helps them withstand the huge force of the hit.
  • Bird eggs are domes too. The curved shell is strong enough for a parent bird to sit on.
  • Spider silk can stretch a long way before it breaks, so a web can catch a flying insect without tearing.
A painted turtle with a dark domed shell and red and yellow markings, basking on a log in a pond.
A painted turtle's domed shell works like an arch in every direction, spreading out a push so it doesn't land on one weak spot. AI-generated image
A dome spreads a pusha big push hereshared around the whole shelland down to the groundturtle shell, egg, arena roof: same idea
A push on top of a dome is spread out around the curved shell instead of landing on one weak spot. That is why a turtle shell and a bird egg are so strong for their size.

Plants: roots, bendy stems and smart shapes

  • Roots spread wide and grow deep. They anchor a tree so wind can't easily push it over or pull it out.
  • Flexible trunks and branches bend in the wind and spring back, instead of snapping. Willows have long, bendy branches.
  • Many grass stems are hollow tubes. A tube is light but strong, and it bends in the wind without breaking.
  • Conifers like spruce and fir have a cone shape with branches that bend down under snow, so heavy snow slides off.
  • Kelp, a large seaweed that grows along Canada's Pacific coast, has a flexible stem that sways with the waves and a rootlike holdfast that grips the rocks.
  • Thick bark protects a tree trunk from bumps and scrapes.
How a tree stands up to windwindflexible trunk and branchesbend with the wind, thenspring back (dashed = upright)deep, spreading rootsanchor the tree in the soil
Two ways a tree withstands wind: its flexible trunk bends instead of snapping, and its roots anchor it so the wind can't push it over. (Bend drawn bigger than in real life.)
Living thingPhysical characteristicHarmful forceHow it helps
turtlehard domed shellcrushing bitedome spreads the push
musselstrong threadspull of wavesthreads hold on in tension
muskoxthick skull and horn basehead-on clashthick, strong layers take the hit
spruce treecone shape, bendy branchesweight of snowsnow slides off
grasshollow, flexible stemwindbends instead of breaking
tree rootsdeep, spreading rootswind pushing the treeanchor the tree in the soil
Physical characteristics that protect living things from forces

Nature gives engineers ideas

Engineers often study plants and animals to get ideas for designs. Domes, hollow tubes and flexible joints are all found in nature and in buildings. This is a great example of science, technology and design working together.

Worked example

Let's match nature to engineering

Match each living thing to a human-built structure that uses the same idea: (a) turtle shell, (b) tree roots, (c) hollow grass stem, (d) mussel threads.

  1. (a) A turtle shell is a dome. Domes are used for roofs of arenas and planetariums.
  2. (b) Roots anchor a tree. A building's foundation anchors it in the ground.
  3. (c) A hollow stem is a tube: light but strong. Bicycle frames and many poles are hollow tubes.
  4. (d) Mussel threads hold on in tension, like the guy wires that hold a tall radio tower steady.

(a) dome roof, (b) foundation, (c) hollow tube frames and poles, (d) guy wires (cables in tension).

Worked example

Let's explain why a willow bends

In a windstorm, a big, stiff branch snaps off a tree, but a willow's long thin branches just bend and sway. Why?

  1. Wind pushes on branches, making them bend (compression on one side, tension on the other).
  2. A stiff branch can't bend much. When the force is too big, it snaps.
  3. A flexible branch bends with the wind, and the force on it gets smaller as it bends out of the way. Then it springs back.

The willow's branches are flexible, so they bend with the wind instead of breaking.

Just for fun

Riddle: Why does a turtle never need an umbrella?

It always carries its own dome roof!

Hands-on activity

Hollow tube test

You need: 2 sheets of the same paper, tape, a stack of books, paper clips (or coins), and a ruler.

  1. Flat strip: fold one sheet in half the long way, then in half again, to make a flat strip.
  2. Hollow tube: roll the other sheet the long way into a tube about 2 cm across. Tape it closed.
  3. Slide one end of the flat strip about 5 cm under a heavy stack of books, so it sticks straight out from the table edge like a branch.
  4. Hang paper clips one at a time from the far end (bend one open to make a hook). Count how many it holds before the end droops 5 cm.
  5. Repeat with the tube.
  6. Bonus: gently bend a fresh blade of grass or a green onion and let go. Then gently bend an uncooked spaghetti noodle. Which springs back? Which snaps?

What to look for: the hollow tube holds many more clips than the flat strip, even though they use the same amount of paper. That is why grass stems, bird bones, bamboo and bicycle frames are tubes. The fresh grass bends and springs back (flexible); the dry spaghetti snaps (stiff and brittle).

Safety: keep paper clips away from younger brothers and sisters.

Your turn

1

What is a physical characteristic?

2

How does a turtle's domed shell protect it?

3

How do roots help a tree withstand wind?

4

Why do spruce trees shed snow well?

5

How do mussels stay on rocks when waves crash over them?

6

Why does grass bend in the wind instead of breaking?

More practice: warm-up

1

Name an animal with a hard shell.

2

What is an exoskeleton?

3

What shape is a bird's egg like?

4

Which part of a tree anchors it?

5

Which is stronger for the same amount of paper: a flat strip or a hollow tube?

More practice: on your own

1

Muskoxen clash heads. Name the physical characteristic that protects them and the force it protects against.

2

Why might a tree with shallow roots fall over more easily in a windstorm?

3

How is spider silk like a good tie in a structure?

4

Name one way kelp is protected from waves.

5

Pick an animal or plant and a human-built structure that use the same idea. Explain the idea.

Challenge

1

Your bones are not solid all the way through: many have a hollow or spongy middle. How might this help you? Use your hollow tube test.

2

Why would a stiff tree survive a light wind but break in a big storm, while a flexible tree survives both?

3

An engineer wants to design a roof for a hockey arena with no columns in the middle. Which natural structure could give an idea, and why?

Check your understanding

Quiz

1.

How does a turtle's dome-shaped shell protect it from a crushing bite?

2.

Which plant characteristic helps a tree stay standing in strong wind?

3.

Why do spruce trees handle heavy snow well?

4.

Mussel threads protect mussels from being washed off rocks by holding on in...

5.

In the tube test, why did the hollow tube hold more clips than the flat strip?

6.

Which animal clashes heads and has a thick skull to protect it?

For parents and teachers

Big idea (D2.4): Plants and animals have physical characteristics that protect them from potentially harmful forces: domed shells, exoskeletons, thick skulls, anchoring threads and roots, flexible and hollow stems, and shapes that shed snow. These connect to structural ideas from engineering (domes, tubes, flexibility, anchors, tension). The tube test practises a fair test (A1.2), and comparing nature and engineering connects science with design (A3.2).

How to help: On a walk, look at how trees bend in the wind, how snow sits on spruce compared with maple branches, or how a snail's or clam's shell is shaped. Ask, "What force is this protecting against? How?"

Common difficulties:

  • Saying "it is strong" without explaining how. Push for the idea behind it: it spreads the force, it bends instead of breaking, it anchors.
  • Thinking stiff is always better. Flexibility is often what saves a plant in strong wind.
  • Safety: keep paper clips away from young children. Only bend fresh plants from your own garden or groceries, not plants in parks.