Every object behaves as if its entire weight pulls down from a single point, its centre of gravity. Where that point sits, and how wide the base beneath it is, decides whether a gentle nudge rights the object or tips it over.
The Key Idea
The centre of gravity is the point at which all of an object's weight can be taken to act. An object topples when the line of action of its weight passes outside its base. A wider base and a lower centre of gravity make an object more stable.
SECTION 01
Tilt it until it tips.
Tilt the block. A vertical line drops from its centre of gravity, representing the line of action of its weight. While that line lands inside the base, the block settles back. Raise the centre of gravity height and watch how dangerously small the toppling angle becomes.
SECTION 02
Making things steady.
Wider base: the line of action can move further before it leaves the base, so a larger tilt is needed to topple it.
Lower centre of gravity: a low, heavy base (like a racing car) needs a much larger tilt before toppling.
Tall and narrow is risky: a high centre of gravity over a small base topples easily.
Finding the centre of gravity of a flat shape
Hang the shape freely from a pin so it can swing, and let it settle.
Hang a plumb line from the same pin and mark the vertical line on the shape.
Repeat from a different hole; the centre of gravity is exactly where the lines cross.
Topples when the weight line leaves the base
An object does not topple simply because it is tilted. It stays stable as long as the line of action of its weight stays within the base. It tips over only once that line passes outside the base, beyond the edge it pivots on. Lowering the centre of gravity or widening the base makes this tipping point much harder to reach.