Types of equilibrium

Stability

Stability is a measure of how likely it is for an object to topple over when pushed or moved.

Stable objects are very difficult to topple over, while unstable objects topple over very easily.

An object will topple over if its centre of gravity is ‘outside’ the base, or edge, on which it balances.

This plumbline is in stable equilibrium because if it is pushed to one side it will return to its original position. It does this because when you push it to one side its centre of gravity rises and tries to pull it back to its lowest position. 

If you carefully balance a ruler on your finger, it is in unstable equilibrium, because if it moves slightly, its centre of gravity falls and keeps falling. 

The football on a perfectly level surface is in neutral equilibrium, because if it is moved, its centre of gravity does not rise or fall.

Activity

What you will need:

Two boxes of matches
a piece of wood

What you will do:
1.  Place the matchbox upright on the piece of wood
2. Slowly tilt the wood until the box topples over.  How high did you tilt the wood?

3.  Put the box on the wood so it has a longer base. Tilt the wood until the box falls. Did you lift it higher or lower than the first time?
4. Raise the centre of gravity by sliding open the second box of matches. Tilt the wood. Is the box more or less stable?

To make the box more stable, should it have:

• A high or low centre of gravity?
• A narrow or wide base?

Han Wern Kuang. (2018). Moments (Standard youTube licence)

For an object to be stable it must have:

• a wide base
• a low centre of gravity

Objects with a wide base, and a low centre of gravity, are more stable than those with a narrow base and a high centre of gravity.

The purple car has a wider wheelbase and lower centre of gravity than the yellow car. It is more stable.

The wheel acts as the pivot for the car. The weight has a turning effect or moment, which causes the car to topple over or fall back.

A double decker bus is stable as it has a:

• low centre of gravity because of its low, heavy engine and heavy bottom deck.
• wide wheelbase.

A traffic cone is stable as it has a:

• low centre of gravity G because of its heavy base.
• wide base.