1.7.2States of Matter
Stability — Cambridge IGCSE Chemistry 0620 Revision Notes & Topical Questions
5 min read 5 parts Lesson 11 of 30 Unit quiz
Curriculum Equation Key قوانين الرقم الهيدروجيني والاتزان الأيوني - pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
Word Equation & Conditions: الرقم الهيدروجيني = -لوغاريتم تركيز أيون الهيدروجين | pH + pOH = 14
Location & Specifications: الاتزان الأيوني، كيمياء 3 ثانوي، التحصيلي، و AP Chemistry.
Section 1 of 5Stable or Unstable
Stability depends on where the centre of mass sits relative to the base, not only on the mass of the object.
Stability
- Stability An object is stable when its centre of mass lies above its base, so it returns to rest without toppling.
Toppling
- Toppling An object topples when its centre of mass lies outside its base, so the weight produces a resultant moment that rotates it.
Base
- Base The area of contact between the object and the surface, where a wider base allows the object to be tilted further before toppling.
Centre of mass height
- Centre of mass height A lower centre of mass makes an object more stable, because the object must be tilted further before toppling.
Section 2 of 5Designing for Stability
Designing for Stability
Cars
- Cars Cars have a low centre of mass and a wide base, so they can tilt a long way before toppling.
Buses and lorries
- Buses and lorries Tall vehicles with high centres of mass topple more easily, so they are advised to avoid bridges and motorways in strong winds.
Bunsen burners
- Bunsen burners A wide base keeps the centre of mass above the base, so the burner stays upright on the bench.
Bowling pins
- Bowling pins Ten-pin bowling pins are tall with a small base, so they topple easily when struck.
Section 3 of 5Tilt and Toppling
Tilting an object shows how far it can lean before it falls over.
1
Tilt the object gradually and observe whether the vertical line through its centre of mass moves beyond the edge of the base.
2
When the vertical line through the centre of mass passes outside the base, the object begins to topple.
3
A low, wide object can be tilted through a larger angle than a tall, narrow object before it topples.
4
Standing an object on its narrow base raises its centre of mass, so it becomes less stable.
Section 4 of 5Worked Example
Question
- A box of width 20 cm and height 60 cm has a uniform density, so its centre of mass is at its geometric centre.
- Compare its stability when it stands on its base and when it lies on its side.
- Standing on its basethe base is 20 cm wide and the centre of mass is 30 cm above the surface.
- Lying on its sidethe base is 60 cm wide and the centre of mass is 10 cm above the surface.
- A wider base and a lower centre of mass both increase stability, and both changes apply when the box lies down.
Answer The box is more stable when it lies on its side, because its base is wider and its centre of mass is lower.
Section 5 of 5Exam Checklist & Tips
1.7.2 – Key Points for the Exam
- Stability The tendency of an object to return to its original position after a small tilt.
- Centre of mass The point at which the whole mass of an object can be considered to act.
- Base The area of an object in contact with the surface it stands on.
- Toppling Falling over, which happens when the centre of mass moves outside the base.
Common Mistakes
- Saying a heavy object is always stable;
- stability depends on the position of the centre of mass and the width of the base.
- Stating that the centre of mass must be at the middle of the base; it must lie above the base area.
- Confusing stability with mass, so a light, low object is wrongly judged less stable than a heavy, tall one.
- Forgetting that tilting an object moves its centre of mass relative to the base.
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