Every object behaves as if its whole weight acts at one point: the centre of gravity. This Core idea explains why racing cars hug the road and why a leaning bus topples, and it gives 0625 one of its favourite describe-an-experiment questions.
What is the centre of gravity?
The centre of gravity is the point at which the whole weight of an object appears to act. For a uniform, symmetrical object (a metre rule, a sphere, a rectangular block), it sits at the geometric centre. For an irregular flat sheet (a lamina), you find it by experiment.
| Term | Meaning |
|---|---|
| Centre of gravity | point where the whole weight appears to act |
| Lamina | thin, flat sheet of material |
| Plumb line | weight hanging on a thread, giving a vertical line |
| Stable equilibrium | object returns to position after a small tilt |
The plumb line experiment has five steps. Make three holes near the edge of the lamina. Hang the lamina from a pin through one hole so it swings freely. Hang a plumb line from the same pin and mark the vertical line on the lamina. Repeat from the other two holes. The lines cross at the centre of gravity; the third line acts as a check.
Why do some objects topple and others not?
An object topples when the vertical line from its centre of gravity falls outside its base. Until that point, the weight creates a moment that turns the object back onto its base. Stability therefore improves with a lower centre of gravity and a wider base. That is the full examiner-approved explanation. Both halves earn marks: the line-through-the-base statement and the low-and-wide design point.
A Bunsen burner has a wide heavy base for exactly this reason. A double-decker bus carries luggage low down. Tilt either one slightly and the weight’s line of action stays inside the base, so it falls back rather than over.
Worked Exam Question
A student is given an irregular flat card, a pin held in a clamp, a plumb line and a pencil. (a) Describe how the student finds the centre of gravity of the card. [4] (b) The card is then balanced flat on a fingertip at the marked point. Explain why it balances there. [2]
Solution (a). Hang the card on the pin through a small hole so it swings freely. Hang the plumb line from the pin and draw the vertical line on the card. Repeat from a second hole. The centre of gravity is where the lines cross; a third hole checks the result.
Solution (b). The whole weight acts at the centre of gravity. The upward force from the finger acts through the same point, so there is no resultant moment and the card balances.
Mark scheme
- B1: card suspended from pin and free to swing.
- B1: plumb line from the same pin; vertical line drawn.
- B1: repeated from a second (and third) hole.
- B1: centre of gravity at the intersection of the lines.
- B1 (b): whole weight acts at that point.
- B1 (b): no resultant moment / support force through the same point.
Common Mistakes
- Writing “the middle of the object” for an irregular lamina. Fix: only uniform symmetrical objects have it at the geometric centre.
- Describing one suspension only. One line cannot locate a point. Fix: state at least two holes, plus a third as a check.
- Forgetting the lamina must swing freely. A clamped card gives a false line. Fix: include “free to swing/rotate” in the description.
- Explaining toppling with “too heavy”. Fix: say the line from the centre of gravity falls outside the base.
- Confusing centre of gravity with the pivot. Fix: the pivot is where it turns; the centre of gravity is where weight acts.
Exam Technique Tip
For stability explanations, write two sentences in this order: first locate the centre of gravity (“low” or “the weight’s line of action stays inside the base”), then state the consequence (“so the moment turns it back onto its base”). Cause then consequence matches the mark scheme split and stops you writing a vague single sentence worth one mark instead of two.
How This Is Examined
Centre of gravity is Core, so both tiers answer it. Papers 1 and 2 use MCQs on stability: which beaker design topples first, where the centre of gravity of an L-shape sits. Papers 3 and 4 ask for the plumb line description (4 marks) or a toppling explanation (2-3 marks), and it slots naturally into 6-mark extended responses with moments. Paper 6 can present the experiment with a diagram and ask for sources of inaccuracy: thick pencil lines, the card not swinging freely, holes too close together. Learn the five steps as a numbered list and you bank those marks in under two minutes.
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