Stability is a measure of how easily an object will topple over when a force is applied to it. It refers to the tendency of a body to return to its original position after it experiences a turning effect.
For example, a pencil on its tip will easily fall over when pushed (it has low stability) whereas a living room sofa will easily fall back into place as it is lifted at one end (high stability).
Factors affecting stability
The two factors which determine the stability of an object are:
- The height of the center of gravity – objects with lower centers of gravity are more stable than those with higher centers
- The width of the base – objects with wider bases are more stable. Of course the axis about which the object is being displaced is important – a sheet of paper may have a wide base along its length or width but it has a very narrow base along its thickness hence why the paper sheet isn’t very stable
Types of Equilibrium
There are three types of equilibrium:
- Stable equilibrium – the object produces forces/torques which return it to its original position after it is displaced by a small force/torque
- Unstable equilibrium – the object produces forces/torques which increase its displacement from its original position after it is displaced by a small force/ torque
- Neutral equilibrium – any small force/torque does not result in any force/torque from the object
Stable Equilibrium
When the object is slightly displaced by a torque, a moment is produced by its center of gravity about the pivoting end of its base to return it to its original position. For example, a traffic cone can be pivoted about any of its four sides by pushing horizontally or by lifting under one of the sides. Assuming the displacement isn’t too great, the center of gravity will remain within its base and create a moment to oppose the initial displacing force.
| Property | Characteristics |
|---|---|
| Center of gravity | Raised by the displacing force/torque, remains within the base |
| Moment | Acts in opposition to the displacing force |
Unstable Equilibrium
When the object is slightly displaced by a torque, a moment is produced by its center of gravity about the pivoting end of its base to further displace it from its original position. For example, the pencil on its tip being pushed will cause its center of gravity to move outside its base and result in a moment which will further topple it.
| Property | Characteristics |
|---|---|
| Center of gravity | Lowered by the displacing force/torque, falls outside of the base |
| Moment | Acts in the same direction as the displacing force |
Neutral Equilibrium
When the object is slightly displaced by a torque, no moment results as the center of gravity does not change position. For example, a pencil on its side or a cricket ball will not have any resulting moment when slightly displaced.
| Property | Characteristics |
|---|---|
| Center of gravity | Does not change in terms of position |
| Moment | None |
Challenge
1) A pencil placed on its side is in stable equilibrium
- True
- False
2) In neutral equilibrium, a displacing force causes the center of gravity of the object to
- remain at the same position
- be raised
- be lowered