Grade by Grade
D. Structures and Mechanisms · Strong and Stable Structures

Stability: Staying Steady, Balanced and Afloat

D2.4 D2.6 A1.2 A1.4About 40 minutes
Learning goals
  • I can explain that a stable structure keeps its shape, stays balanced, floats upright or stays in one spot when a force pushes on it.
  • I can describe ways to make a structure more stable: a wide base, heavy parts low, anchors and braces.
  • I can explain what helps a boat float steadily.
  • I can test a tower and a boat to find out what makes them stable.

Words to know

stable
Steady. A stable structure keeps its shape, stays balanced and stays in place when a force pushes on it.
stability
How well a structure stays steady and does not tip, wobble, change shape or move away when a force acts on it.
base
The bottom part of a structure that rests on the ground.
centre of gravity
The balance point of an object, where its weight seems to be centred. A low centre of gravity makes things harder to tip over.
anchor
Something that holds a structure in place, like a tent peg, a heavy weight or a foundation in the ground.
foundation
The strong base of a building, built into the ground, that holds it steady.

Strong is not enough!

Imagine a very strong bookshelf made of thick wood. It can hold 100 books. But it is tall and thin, and nobody fastened it to the wall. A child climbs it, and... it tips over!

That shelf was strong, but not stable. A good structure must be both.

Remember from Grade 1

In Grade 1 you saw that a pyramid of blocks with a wide bottom is harder to knock over than a tall, thin tower. In Grade 3 you will learn four ways a structure must be stable, and lots of ways to make it more stable.

What is stability?

Stability is a structure's ability to stay steady when a force is applied to it (when something pushes or pulls on it). A stable structure:

  1. Keeps its shape. It does not sag, lean or fold up.
  2. Keeps its balance. It does not tip or fall over.
  3. Floats upright, if it is on water. It does not roll over or sink.
  4. Stays fixed in one spot. It does not slide or blow away.
A red canoe floats upright on a calm lake beside a wooden dock, with pine trees on the rocky shore.
A canoe must float upright without rolling over. That is one of the four ways a structure must be stable. AI-generated image
narrow basewide baseboat floats
A tall, narrow tower tips easily. A shape with a wide base is stable. A boat with a wide bottom floats upright.

Ways to make a structure more stable

  • Make the base wider. A wide base is harder to tip. Think of a traffic cone or a lamp with a big, flat bottom.
  • Put heavy parts low. When the heaviest parts are near the bottom, the centre of gravity (the balance point) is low, and the structure is hard to tip. Put heavy books on the bottom shelf.
  • Anchor it. Hold it in place with tent pegs, sandbags, bolts, or a foundation dug into the ground. Tall furniture can be fastened to the wall.
  • Balance the load. Spread the load evenly on both sides, so it does not lean one way.
  • Brace it. Diagonal braces (struts and ties) keep a frame from changing shape.

Staying stable on water

Boats, rafts and docks are structures too. To be stable on water, a structure must float and stay upright.

  • A wide, flat bottom makes a boat steadier.
  • Keep the load low and in the middle. That is why people in a canoe sit or kneel low and don't stand up: standing raises the balance point, and the canoe can tip.
  • Spread the load evenly from side to side and front to back.
  • Some boats add an outrigger, a float on the side, to keep from tipping.
Worked example

Let's improve a tippy tower

Zoe builds a tower of blocks 30 cm tall with a base of just one block. It falls when she bumps the table. How can she make it more stable?

  1. Widen the base: start with 4 blocks in a square, then make the tower narrower as it goes up, like a pyramid.
  2. Heavy parts low: use the biggest, heaviest blocks at the bottom and small, light ones at the top.
  3. Anchor it: put a little modelling clay under the bottom blocks so they grip the table.
  4. Test: bump the table the same way as before. Does it stay up now?

A wider base, heavy parts low and an anchor make the tower much more stable.

Worked example

Let's think about a canoe

Why do people say "Stay low and in the middle" when you get into a canoe?

  1. A canoe is long and narrow, so it can roll over sideways.
  2. If you stand up, your weight is high up. The balance point goes up, and a small lean can tip the canoe.
  3. If you lean to one side, the load is not balanced, and that side goes down.
  4. Sitting or kneeling low and in the middle keeps the balance point low and centred.

Staying low and centred keeps the canoe stable, so it floats upright.

balance pointis HIGHStanding up: TIPPYa small lean can roll it overbalance pointis LOWKneeling low, in the middle:STEADY
A canoe seen from the end. Standing up raises the balance point, so the canoe tips easily. Kneeling low in the middle keeps it steady.
Just for fun

Why are pyramids never nervous?

They have a wide base and keep their heavy parts low. Nothing knocks them over!

Safety first

Tall furniture like bookshelves and dressers should be fastened to the wall by a grown-up so they cannot tip. Never climb on shelves. In a boat or canoe, always wear a life jacket and stay seated.

Hands-on activity

Stability lab: towers and boats

Part 1: Tower test. You need: paper or plastic cups (or blocks), a piece of cardboard for a fan, and modelling clay.

  1. Build a tower 1 cup wide and 6 cups tall. Fan it gently with the cardboard from 30 cm away. Does it fall?
  2. Build a tower with the same 6 cups but a wider base (3 on the bottom, 2, then 1). Fan it the same way.
  3. Put a lump of clay in the bottom cups. Test again.

Part 2: Boat test. You need: two squares of aluminum foil (15 cm by 15 cm), a sink or tub with water, and about 30 coins.

  1. Shape one square into a boat with a wide, flat bottom and low sides. Shape the other into a narrow, tall boat.
  2. Float each one. Add coins one at a time to the middle, spread out. Count how many each holds before it tips or sinks.
  3. Now try putting all the coins on one side of the wide boat. What happens?

What to look for: the wide-base tower and the clay-weighted tower stand up to the fan best; the wide, flat boat holds more coins and stays upright longer; coins piled on one side make the boat tip. Safety: a grown-up stays nearby whenever there is water; wipe up spills so no one slips; keep coins away from little children.

Your turn

1

What does stable mean?

2

Which is more stable: a tall tower with a narrow base or a short tower with a wide base?

3

Where should you put the heaviest books on a bookshelf: top or bottom? Why?

4

What do tent pegs do for a tent?

5

Why should you not stand up in a canoe?

6

Can a structure be strong but not stable? Give an example.

7

A sign at a garage sale keeps blowing over. Give two ways to make it more stable.

More practice: warm-up

1

Is a traffic cone stable? What helps?

2

What do we call the bottom part of a structure?

3

What holds a house steady in the ground?

4

Should the load on a raft be spread out or all on one edge?

5

Name one way a structure can be unstable.

More practice: on your own

1

Why do many lamps and fans have a big, heavy base?

2

A snow fort wall keeps falling over. It is tall and only one block thick. Give two ways to fix it.

3

Why do builders dig a deep foundation for a tall building?

4

In the foil boat test, why must both boats start as the same size square of foil?

5

Your foil boat tipped when you put all the coins on one side. Explain why.

Challenge

1

A backpack hanging on the back of a chair makes the chair tip backward. Explain why, using the words balance and load.

2

Inuit build stone landmarks called inuksuit (one is an inuksuk) by stacking and balancing stones with no glue. What helps an inuksuk stay standing in strong Arctic winds?

3

Design a dock for a lake that will stay stable when people walk on it and waves push on it. Name three features.

Check your understanding

Quiz

1.

A stable structure...

2.

Which makes a tower more stable?

3.

Where should heavy things go on a shelf?

4.

Tent pegs keep a tent...

5.

Which foil boat is most likely to float steadily?

6.

In a canoe, it is safest to...

For parents and teachers

Big idea: Stability is a structure's ability to keep its shape, keep its balance, float upright and stay in one spot when a force is applied. Stability improves with a wide base, heavy parts low (a low centre of gravity), anchors and foundations, balanced loads and bracing (D2.4, D2.6). The tower and boat tests are fair tests (A1.2) with safety rules for water (A1.4).

How to help: Point out stable designs: traffic cones, lamp bases, tent pegs, sandbags on signs. Check together whether tall furniture in your home is anchored to the wall. In the bath or sink, float different containers and ask why some tip.

Common difficulties:

  • Thinking strong and stable mean the same thing. Strong = holds a heavy load; stable = stays steady and does not tip, sag or move.
  • Thinking heavier is always less stable. Weight placed low actually helps stability.
  • Forgetting floating. Boats and docks must stay upright on water.

Safety: always supervise water play; wipe up spills; keep coins away from small children. In any boat, everyone wears a life jacket.