Grade by Grade
D. Structures and Mechanisms · Form and Function

Stability and the Centre of Gravity

D2.2 A1.2 A1.5 A3.1About 45 minutes
Learning goals
  • I can explain how the position of the centre of gravity affects a structure's stability.
  • I can explain why a low centre of gravity and a wide base make a structure more stable.
  • I can describe ways people make structures more stable, such as counterweights, anchors and wide bases.

Words to know

stability
A structure's ability to stay balanced and upright, and return to its position after being pushed, instead of tipping or falling over.
stable
Hard to tip over; returns to its position after a small push.
unstable
Easy to tip over.
base of support
The area on the ground outlined by all the places where a structure touches it.
counterweight
A heavy weight placed on the opposite side of a structure to balance a load and keep the CG over the base.
anchor
Something that holds a structure firmly to the ground or a wall, like bolts, pegs or straps.

Hook: the tipping test

Stand a full cereal box upright on the table and give it a gentle push near the top. It falls over. Lay the same box flat and push the same way. It just slides.

Same box. Same mass. Same push. The difference is where its centre of gravity is compared with its base.

Two identical cardboard boxes on a table: one standing upright on its narrow end and one lying flat on its wide side.
Same box, same mass. The upright box has a high centre of gravity over a small base, so it tips easily; the flat box has a low centre of gravity over a wide base. AI-generated image

The tipping rule

Imagine a line straight down from the centre of gravity (like a plumb line).

  • If that line lands inside the base of support, the structure is stable. If it is tilted a little and let go, it falls back.
  • If the line lands outside the base, the structure tips over.

So a structure is more stable when it is hard to move its CG line outside its base. Two things help most:

  1. A low centre of gravity. The lower the CG, the more you have to tilt the structure before the CG line passes the edge of the base.
  2. A wide base. The wider the base, the farther the CG line can move before it reaches an edge.
CG line inside the base:falls back (stable)CG line outside the base:tips over
Both boxes are tilted by the same angle on their right corner (the small circle). The wide, low box's CG line still lands inside its base, so it falls back. The tall, narrow box's CG line lands outside its base, so it tips over.

Ways to make structures more stable

  • Widen the base: traffic cones, tripods, a hockey player's wide stance, the spread legs of a ladder.
  • Lower the CG: put heavy parts at the bottom. Heavy books on the bottom shelf; a heavy base on a floor lamp; sitting low in a canoe.
  • Counterweights: a tower crane has a big concrete counterweight on its short arm, opposite the load. A seesaw balances when the heavier person sits closer to the middle.
  • Anchors and foundations: tent pegs, bolts into concrete, deep foundations, and the anti-tip straps that attach tall furniture and TVs to the wall.
  • Guy wires: cables staked into the ground on different sides help hold up tall, thin towers and flagpoles.

Workers in many jobs use these ideas every day: crane operators check loads and counterweights, carpenters and installers anchor cabinets, and movers load heavy items low in the truck.

Worked example

Let's compare two designs

Two backyard birdhouse stands are the same height. Stand A has a heavy concrete base 40 cm wide. Stand B sits on a light plastic base 15 cm wide. Which is more stable in wind? Explain.

  1. Stand A's heavy base makes its CG lower.
  2. Stand A's base is also wider (40 cm vs 15 cm).
  3. For Stand A to tip, the wind would have to tilt it a lot before its CG line moves outside its wide base.
  4. Stand B's CG is higher and its base is narrow, so a small tilt moves the CG line outside the base.

Stand A is more stable: lower CG and wider base.

Worked example

Let's explain a crane's counterweight

A tower crane lifts a 2 000 kg load at the end of its long arm. Why doesn't it tip over?

  1. The load on the long arm pulls the combined CG toward the load, away from the centre of the tower's base.
  2. The crane has a large counterweight near the end of its short arm, on the other side.
  3. The counterweight pulls the combined CG back over the base.
  4. The crane is also anchored to a big concrete foundation.

The counterweight keeps the combined centre of gravity over the base, and the foundation anchors it.

concrete foundationcounter-weightloadcombined CGstays over the baseshort armlong arm
A tower crane's counterweight on the short arm balances the load on the long arm, so the combined centre of gravity stays over the base. The foundation anchors it too.

Home safety connection

Tall dressers, bookcases and TVs can tip over if a child climbs them or pulls out the drawers, because that moves the CG forward past the front edge of the base. Ask a grown-up if your tall furniture is anchored to the wall with anti-tip straps or brackets.

Just for fun

Why are traffic cones so hard to knock over?

A wide base, a low centre of gravity... and years of practice just standing around.

Hands-on activity

Stability lab: tilt angle

You need: an empty cereal box, a few heavy cans or bags of rice or dried beans, a ruler, a protractor (or a phone's level app), tape.

  1. Stand the empty box upright. Slowly tilt it on one bottom edge until it tips over by itself. Measure the tilt angle just before it tips (use the protractor, or ask a helper to read it). Repeat 3 times and find the average.
  2. Put the heavy cans or a bag of rice at the bottom inside the box (lower CG). Tape it closed. Repeat the tilt test 3 times.
  3. Move the heavy items to the top: open the box, tape the bag of rice or the cans firmly to the inside of the box near the top, and close it. Repeat 3 times.
  4. Lay the box on its widest side (wider base). Repeat.
  5. Make a table of average tilt angles and write a conclusion using the words centre of gravity, base and stable.

What to look for: weight at the bottom gives the biggest tilt angle (most stable), weight at the top gives the smallest, and the wide side is the most stable of all. Do the test on the floor, away from anything breakable.

Your turn

1

What does stability mean?

2

State the tipping rule.

3

Name two ways to make a structure more stable.

4

Why should heavy books go on the bottom shelf?

5

What is a counterweight?

6

Why does a hockey goalie crouch with feet wide apart?

7

Why is a tall, narrow box easier to tip than a short, wide one?

More practice: warm-up

1

Is a low CG more stable or less stable?

2

Is a wide base more stable or less stable?

3

Name the part of a tower crane that keeps it from tipping toward the load.

4

What do tent pegs do?

5

Name an object at home with a heavy base to keep it stable.

More practice: on your own

1

A loaded camping trailer has all its heavy gear packed on the roof rack. Why is this a bad idea?

2

Explain why a ladder's legs spread apart at the bottom.

3

Why do tall radio towers often use guy wires?

4

Two people sit on a seesaw. One is much heavier. How can they balance it?

5

A child pulls out all the drawers of a tall dresser and it starts to tip. Use the tipping rule to explain.

Challenge

1

In the picture, both boxes are tilted 25°. Explain, using the CG line, why one falls back and the other tips.

2

An engineer is designing a lifeguard chair for a beach. It must be tall. Suggest three design features to keep it stable, with reasons.

3

Explain why a moving truck with a high load of furniture should take corners slowly.

Check your understanding

Quiz

1.

A structure tips over when the line down from its CG...

2.

Which is the most stable?

3.

Why does a tower crane have a counterweight?

4.

Which change makes a bookshelf less stable?

5.

Guy wires help a tall tower by...

6.

Why do paddlers kneel or sit low in a canoe?

For parents and teachers

Big idea (D2.2): A structure is stable when the line straight down from its centre of gravity falls inside its base. A low CG and a wide base make structures more stable; counterweights, anchors and guy wires also help. Workers like crane operators use these ideas daily (A3.1).

How to help: Point out tipping risks at home. This is a great time to check that tall furniture and TVs are anchored to the wall. Ask your child to explain why using the words centre of gravity and base.

Common difficulties:

  • Thinking heavier always means more stable. Weight placed high makes things less stable.
  • Mixing up strong and stable. A strong structure doesn't break; a stable one doesn't tip.
  • Lab safety: tilt tests should be done on the floor, away from breakable things, with cans that can't roll off a table.