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 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.
Definition · learn the exact words
The centre of gravity is the point at which all the weight of an object can be taken to act.
topples when the weight line leaves the base
More stable means harder to topple: a wider base and a lower centre of gravity.
Section 01
Tilt it until it tips.
Tilt the object and watch the line of action of its weight. While that line stays inside the base, the object rocks back upright. Push it far enough that the line crosses the edge, and it topples.
Stage 1 · Learn
Check stability and toppling
Four quick checks on the centre of gravity, toppling, and stability. Each correct answer earns XP and lights this skill on your star map.
Quick check+10 XP
The centre of gravity of an object is...
Quick check+10 XP
An object topples over when...
Quick check+10 XP
Which change makes an object more stable?
Quick check+10 XP
To find the centre of gravity of an irregular sheet of card (a lamina), you use...
Section 02
Stable, until the line escapes.
Where the weight acts: treat the whole weight as acting straight down through the centre of gravity.
The tipping test: while the line of action of the weight stays inside the base, a tilt creates a moment that rights the object.
Designing for stability: a lower centre of gravity and a wider base both push the tipping point further away.
Worked example · required practical
Describe how to find the centre of gravity of an irregular sheet of card (a lamina).
Step 1 · Hang it freelyMake a small hole near the edge and hang the card from a pin so it can swing and settle.
Step 2 · Mark the verticalHang a plumb line from the same pin and draw the vertical line it makes on the card.
Step 3 · Repeat and crossRepeat from a second hole. The centre of gravity is the point where the two lines cross.
Examiner trap
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.
Stage 2 · Exam
Exam-style questions
Unlocks once the four checks above are done. Worth more XP, written in the style of Paper 1.
Finish the four checks above to unlock the exam questions
Exam style+20 XP
A racing car is designed to be very stable. The best design features are...
Exam style+20 XP
A box is tilted on one edge and then released. It will fall back upright as long as...
Exam style+20 XP
For a uniform, symmetrical object such as a metre rule, the centre of gravity is at...
Skill unlocked
Centre of gravity, mastered.
This skill is now lit gold on your star map. Keep the chain going.
Mixed questions across the whole unit, each one worth XP. Start this only when you feel confident across every topic in the unit, not just the centre of gravity.