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Cambridge IGCSE Physics · 0625

Chapter 1: Motion, forces and energy — Part 7

Topic 1.5.3 · Centre of gravity

Centre of gravity

The centre of gravity is the point where the weight of an object appears to act.

For a uniform regular shape it is at the geometric centre. For an irregular thin sheet (lamina) it must be found by experiment.

Finding the centre of gravity of a lamina

  • Punch a hole near the edge of the card and suspend it so it can swing freely.
  • Hang a plumb line from the same pin and mark the vertical line on the card.
  • Repeat from a different hole.
  • The centre of gravity is where the two lines cross.
Finding the centre of gravity of an irregular card lamina: suspend from two different holes, hang a plumb line each time, and mark where the lines cross.
Diagram 1: Finding the centre of gravity of an irregular card lamina: suspend from two different holes, hang a plumb line each time, and mark where the lines cross.

Stability

An object is more stable if its centre of gravity is low and its base is wide.

If the vertical line from the centre of gravity falls inside the base, the object returns to upright. If that line falls outside the base, it topples.

Stability: a wide-base low centre of gravity object that stays upright versus a tall narrow object whose weight line falls outside the base when tilted, so it topples.
Diagram 2: Stability: a wide-base low centre of gravity object that stays upright versus a tall narrow object whose weight line falls outside the base when tilted, so it topples.

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