Stability increases when the centre of mass is low, the base of support is wide, and the line of gravity stays well inside that base. Greater mass and friction can also resist displacement.
The centre of mass is the point where mass is considered concentrated. The line of gravity extends vertically downward from it. The base of support is the area enclosed by all contact points. Balance is maintained while the line remains inside this area.
| Factor | Effect on stability | Sporting application |
|---|---|---|
| Height of centre of mass | Lowering it increases stability because more rotation is needed before the line leaves the base. | A wrestler crouches to resist a throw. |
| Size of base of support | A wider base allows greater movement before the line crosses an edge. | A rugby player widens their stance. |
| Position of line of gravity | Stability is greatest near the centre and decreases toward an edge. | A sprinter leans forward before starting. |
| Body mass | Greater mass requires greater force to produce the same acceleration. | A heavier athlete resists displacement. |
| Friction | Greater friction reduces sliding at the contact points. | Studs improve grip on grass. |
| External force | Force direction and application point can create a turning effect. | A high tackle can rotate a player. |
Instability occurs when the line of gravity passes beyond the base. An external force may then produce a turning moment, , where is the perpendicular distance from the pivot.
A common misconception is that a low centre of mass guarantees stability. It does not: once the line of gravity leaves the base, rotation or falling can occur.
In an IB exam, define the three key terms, explain their relationship, and apply them to a named movement. For explain, link cause and effect using Newton's laws or turning moments.