IGCSE Physics / The turning effect of forces
Centre of gravity and stability revision notes
Edexcel International GCSE Physics (4PH1), chapter 5: The turning effect of forces, specification point 1.31P. Free lesson.
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The centre of gravity is the point through which an object's weight acts. Its position and the width of its base determine how far it can tilt before it topples.
What the centre of gravity means
Every part of an object has weight, but you can treat the whole weight as acting at one point. This point is the centre of gravity. For a regular shape, such as a rectangle, circle or uniform rod, it is at the centre.
How to find the centre of gravity of a flat card
For an irregular card, use a pin and a plumb line. The line hangs vertically, so the centre of gravity must lie on the line drawn along its thread. Two lines from different hanging points show where the centre of gravity is. A plumb line is a thread with a small weight on the end.
When an object topples
Imagine a vertical line of action going down from the centre of gravity. When the object is upright, this line falls through the middle of its base. If the object tilts a little, the line may move away from the middle but still stay inside the base. The weight then creates a moment about the edge that turns the object back. It topples only when the line passes outside the edge of the base.
What makes an object stable
Stability means how hard an object is to topple. A low centre of gravity and a wide base both make an object more stable. With these features, the object can tilt further before the line of its weight passes outside the base. A racing car is low and wide; a tall, thin vase is less stable. Low centre of gravity: more stable. High centre of gravity: less stable, because a small tilt can take the line of weight outside the base. Wide base: more stable. Narrow base: less stable, because the line of weight has less distance to travel before it leaves the base. A more stable object topples only…