Stable objects have both a broad base of support and a low centre of gravity. The centre of gravity of an object could also be called its balance point. If you support the centre of gravity, the object will balance and be stable. If an object is not supported directly below its centre of gravity then the object will be unstable and topple over.

Any object is most stable when the centre of gravity is near the centre of the base of support. Most structures, whether they are buildings or people, fall down because they are unstable, not because they are weak.

We can make objects more stable just by arranging them in a particular way. Objects with a small base are less stable than objects with a large base. For example, a pyramid is much more stable sitting on its broad base than on its point.

For any object that has a supporting base, its centre of gravity must be located directly over the base or it will tip over. This explains why a wide stance helps you keep your balance.

In order for you to stand up straight and not fall over, your centre of gravity must be directly above your base. Your base is your feet. As you move your centre of gravity (near your belly button) to and fro, it stays above your support (your feet).

When your feet are together, you cannot lean very far to the left or right since your centre of gravity won't be supported. When your feet are apart, you can lean further to the left or right since your base is much wider, supporting the shifting centre of gravity above. When your centre of gravity is no longer above your feet, you topple over.

A tall object with a high centre of gravity and a skinny support will fall easily, because even a little sideways motion pushes the centre of gravity off the skinny support.

For a fun extension: defy gravity with balance! Watch the video that explains this bottle rack!

### Objectives

• Find the centre of gravity of an object.

• Identify ways in which balance affects our daily lives.

• List the conditions required for an object to be stable.

### Materials

• Per Demo or Group:
1 volunteer human
white/blackboard or large paper sheet
marker
sticker dot (optional)

### Key Questions

• Was the volunteer feeling more or less stable throughout the the demo?
• Where is the volunteer’s centre of gravity? Where is their base of support?
• Why is the person more stable when they widen their feet? Why don’t they fall over?
• What does your body want to do when you lean over to one side or another?
• What causes objects to topple over, or lose balance?

### What To Do

1. Invite a volunteer to stand at the front of the class.
2. Draw a caricature of the student on the board. Ask students to identify the centre of gravity and base of support. Alternatively, you could stick a bright coloured paper circle to the student’s belly to represent the centre of gravity.
3. Ask the volunteer to stand on one foot with thier hands by their sides. Ask them to lean over sideways as much as possible without moving their arms.
4. Ask the volunteer to stand with their feet close together. Ask them to lean over as much as possible without moving their arms.
5. Ask the volunteer to stand with their feet wide apart (approximately 0.5 m). Ask them to lean over as much as possible without moving their arms.

### Extensions

• Get the whole class to try different stances to see how their balance is affected.
• Can you bend farther forward than you can sideways? How can you explain that?
• A bottle rack can seem to defy common sense. Why doesn’t the bottle fall over? Hint: Guestimate the location of its centre of gravity. The centre of gravity is at the point of the bottle directly above the base of support of the rack on the table. It does not tip over because its centre of gravity is directly above its base support.

### Other Resources

IISER Pune Sciecne ActivityCenter | Balancing Bottle

Survivors

Artist: Jeff Kulak

Jeff is a senior graphic designer at Science World. His illustration work has been published in the Walrus, The National Post, Reader’s Digest and Chickadee Magazine. He loves to make music, ride bikes, and spend time in the forest.

Egg BB

Artist: Jeff Kulak

Jeff is a senior graphic designer at Science World. His illustration work has been published in the Walrus, The National Post, Reader’s Digest and Chickadee Magazine. He loves to make music, ride bikes, and spend time in the forest.

Comet Crisp

Artist: Jeff Kulak

Jeff is a senior graphic designer at Science World. His illustration work has been published in the Walrus, The National Post, Reader’s Digest and Chickadee Magazine. He loves to make music, ride bikes, and spend time in the forest.

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Artist: Michelle Yong

Michelle is a designer with a focus on creating joyful digital experiences! She enjoys exploring the potential forms that an idea can express itself in and helping then take shape.

Buddy the T-Rex

Artist: Michelle Yong

Michelle is a designer with a focus on creating joyful digital experiences! She enjoys exploring the potential forms that an idea can express itself in and helping then take shape.

Geodessy

Artist: Michelle Yong

Michelle is a designer with a focus on creating joyful digital experiences! She enjoys exploring the potential forms that an idea can express itself in and helping then take shape.

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Artist: Ty Dale

From Canada, Ty was born in Vancouver, British Columbia in 1993. From his chaotic workspace he draws in several different illustrative styles with thick outlines, bold colours and quirky-child like drawings. Ty distils the world around him into its basic geometry, prompting us to look at the mundane in a different way.

Western Dinosaur

Artist: Ty Dale

From Canada, Ty was born in Vancouver, British Columbia in 1993. From his chaotic workspace he draws in several different illustrative styles with thick outlines, bold colours and quirky-child like drawings. Ty distils the world around him into its basic geometry, prompting us to look at the mundane in a different way.