Why a plumb line predicts a tip
For a stationary object resting on a level surface, the key question is where its center of mass sits relative to the area of contact with the ground. The center of mass is the point at which the object’s mass can be treated as concentrated for analyzing gravity. Imagine a plumb line—a line that hangs straight down—from that point. If it falls within the object’s supporting footprint, the ground can push upward in a way that balances the object. If it falls beyond an edge, the object cannot remain in static balance and begins to rotate.
The edge becomes a pivot
As an object starts to lean, the edge of its base can act as a pivot. Gravity pulls downward through the center of mass, and because that line of pull no longer passes through the support, it produces a turning effect, or torque, around the edge. The farther the center of mass shifts past the edge, the stronger that tipping tendency becomes. Once the object has begun rotating, gravity generally carries it farther over rather than restoring it upright.
The same geometry explains why a broad stance is more stable: it gives the center of mass more room to move before its downward line crosses the boundary. Lowering the center of mass can help too, because a larger shift may be needed to move its projection beyond the base.
A useful rule, not the whole story
This test describes the tipping threshold for an object at rest under gravity on a firm, level surface. A shove, a moving base, or a soft or uneven surface can add forces that change the outcome. But for a standing cabinet, a leaning stack, or a person holding still, the plumb-line picture reveals the essential mechanism: tipping begins when gravity’s line of action passes outside the support.
