Describe, qualitatively, the effect of the position of the centre of gravity (CG) on the stability of simple objects.
Key Concepts
The centre of gravity is the point at which the total weight of an object can be considered to act.
Stability refers to an object’s ability to return to its original position after being disturbed.
Two main factors influence stability:
Vertical position of the CG (high vs low).
Horizontal position of the CG relative to the base of support.
Qualitative Effect of CG Position
When an object is tilted, a restoring torque tries to bring it back to equilibrium. The magnitude of this torque depends on the horizontal distance between the CG and the pivot point.
\$\tau = W \times d\$
where \$W\$ is the weight of the object and \$d\$ is the horizontal distance from the CG to the pivot.
Low CG:\$d\$ is small, so the restoring torque is modest but the object’s base is wide relative to the CG, making it hard to tip. The object is stable.
High CG:\$d\$ becomes larger for the same tilt angle, producing a larger restoring torque that can quickly exceed the frictional resistance, causing the object to tip over. The object is less stable or unstable.
Examples of Simple Objects
Object
CG Position
Stability
Typical Use
Baseball bat (standing on a flat surface)
High (near the barrel)
Unstable – easily tipped
Requires a holder or a low‑lying support
Table
Low (near the legs)
Stable – resists tipping
Supports heavy loads safely
Inverted pyramid (e.g., a toy)
High (near the tip)
Unstable – will fall over
Demonstrates centre of gravity concepts
Wheeled shopping trolley
Low (below the handle bar)
Stable when stationary, but can become unstable when accelerating
Design balances CG for safe handling
How to Improve Stability
Lower the CG by adding weight at the base (e.g., sandbags on a crane).
Widen the base of support (e.g., a broader table top).
Design shapes that naturally keep the CG low (e.g., a pyramid with a broad base).
Common Misconceptions
“A heavier object is always more stable.” – Stability depends on the CG position, not just weight.
“If an object does not move, its CG must be at the centre of the base.” – The CG can be anywhere within the base; as long as it stays vertically above the base, the object remains in equilibrium.
Suggested diagram: Side view of a rectangular block showing a low CG (stable) and a high CG (unstable) with arrows indicating the restoring torque for a small tilt.
Summary Checklist
Identify where the CG is located in a given object.
Compare the height of the CG to the width of the base.
Predict whether the object will be stable, unstable, or conditionally stable.
Suggest practical ways to modify the design for greater stability.