An object is stable when a small tilt does not make it fall over. A wider base and a lower centre of mass make an object more stable, and it topples when the vertical line from the centre of mass falls outside the base.
This lesson applies the idea from the centre of mass and completes the core ideas of moments and stability.
Why does the line from the centre of mass matter?
The weight acts downwards through the centre of mass. When you tilt an object about an edge of its base, that edge is the pivot. The weight then has a moment about the edge.
If the vertical line through the centre of mass is still inside the base, the weight’s moment tries to turn the object back. If the line has moved outside the base, the weight’s moment turns the object over.
How do you write a good stability explanation?
- Name the centre of mass and say where it is (high or low).
- Name the base and say whether it is wide or narrow.
- Describe the tilt: the object pivots about an edge of its base.
- State the condition: it topples once the vertical line from the centre of mass falls outside the base.
- Finish with the comparison the question asks for.
Worked example
A uniform block is 0.40 m wide and 0.90 m tall. It rests on its 0.40 m wide base.
Another uniform block is 0.40 m wide and 0.40 m tall. Compare how far each must be tilted before it topples.
For a uniform block, the centre of mass is at the middle of the block. The block tilts about one bottom edge. It is on the point of toppling when the centre of mass is directly above that edge.
Tall block: the centre of mass is 0.45 m above the base and 0.20 m from the edge horizontally. The tilt angle θ satisfies tan θ = 0.20 ÷ 0.45 = 0.444, so θ is about 24°.
Square block: the centre of mass is 0.20 m high and 0.20 m from the edge. tan θ = 0.20 ÷ 0.20 = 1, so θ = 45°.
Answer: the tall block topples after about 24° of tilt, the square block after 45°. The tall block is less stable because its centre of mass is higher relative to the width of its base.
The angle calculation is an illustration of the idea. In exams, you mainly need to explain the reason in words.
The mistake to watch for
A common myth is that a heavier object is always more stable.
Mistaken explanation: “The tall block falls over easily because it is heavier.”
Mass does not decide toppling. Two blocks of the same shape and size but different mass tip at the same angle, because the centre of mass is in the same place.
The correction is to talk about the position of the centre of mass and the width of the base. Adding weight low down can help, because it lowers the centre of mass.
Check yourself
1. Why is a racing car built wide and low?
Show answer
The low centre of mass and wide wheelbase mean the vertical line from the centre of mass stays inside the base even when the car leans hard in a corner, so it is hard to topple.
2. A loaded bus has heavy luggage on the roof. Explain why this makes it less stable.
Show answer
The luggage raises the centre of mass. The bus then needs a smaller tilt before the vertical line from the centre of mass falls outside its base, so it topples more easily.
3. A cone can rest on its base, on its tip or on its side. Name the type of equilibrium in each position.
Show answer
On its base: stable. On its tip: unstable. On its side: neutral, because it stays in its new position when rolled.
Where this leads next
The remaining lesson is checking perpendicular distance in a turning problem, which makes sure every moment you calculate uses the right distance. Then test yourself with the moments and stability practice set.
Stability answers need precise wording, and a teacher in online one-to-one Physics tuition can read your written explanations and show where a missing link loses marks.