Compare low centres of gravity and broad bases
Explain useful stability changes while keeping other dimensions and conditions comparable.
Exam scope and assessment notes
Mapped to the named 0973 section using teaching from the corresponding single-science course. A 0973-specific review of every Core and Supplement objective is still in progress. Written practice is self-checked; it does not demonstrate supervised laboratory competence or award an official Cambridge grade. This qualification reports two grades on the 9–1 scale; confirm the entry route for your zone and exam series.
Before you begin
- Recognise restoring and toppling weight moments.
- Compare objects while stating which features stay fixed.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
What is compared with the base when assessing ordinary static balance?
What you will learn
- Describe how lowering centre of gravity affects stability.
- Compare base width and centre height without unsupported guarantees.
Compare one feature under controlled conditions
For otherwise comparable objects with the same base, lowering the centre of gravity means a larger tilt is needed before its vertical weight line reaches a tipping edge. It therefore improves resistance to toppling in that comparison. For the same centre-of-gravity position and height, a broader base places the edges farther from the initial weight line and also increases the available tipping margin.
A wide-bottomed stand and low-mounted heavy equipment illustrate these ideas. A high load on a vehicle raises the combined centre of gravity relative to the vehicle and can reduce its toppling margin compared with the same load carried lower. The conclusion concerns the combined mass distribution and contact base, not simply how tall the empty outline looks.
Do not replace comparisons by absolute slogans
A low centre of gravity alone does not guarantee that an object never topples: the base might be very narrow, an external moment might be large, or support might shift. Likewise a wide base does not imply unlimited stability under every force. When both base width and centre height change, qualitative single-feature comparisons alone may be insufficient to rank two objects.
For a comparison at rest, identify the supported footprint and where the weight line falls within it. For an ideal tipping comparison, state which edge is the pivot and what other conditions remain similar. The syllabus asks for qualitative stability effects here; no critical-angle formula or certified safety rating follows from these written exercises.
Sources for this lesson
Pause and explain
Does a lower centre of gravity guarantee stability if the base also becomes much narrower?
Worked example
Two otherwise identical stands have the same base, but one has its centre of gravity lower. Which has the greater ideal toppling margin and why?
Show the worked solution
- The bases and other stated conditions are the same.
- A lower centre requires more tilt before its weight line reaches an edge.
- The lower-centre stand has the greater toppling margin under those controls.
Answer The lower-centre stand, because more tilt is needed to bring its weight line to the same base edge.
Try a topic test
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Test this topic →From guided practice to a new situation
Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Compare two otherwise identical objects with the same base but different centre heights.
Give me a hint
Keep the base fixed and follow the weight line during a tilt.
Compare my reasoning
- The lower-centre object starts with the same base edges.
- Its weight line needs a larger tilt to reach an edge.
- It has greater ideal resistance to toppling in this controlled comparison.
The lower-centre object is more resistant to toppling when the base and other conditions are the same.
Look for these in your work
- State the common base.
- Explain the change in weight-line margin.
At the same centre-of-gravity height and position, compare a narrow and broad supported base. Then explain carrying a fixed heavy load lower on the same vehicle.
Give me a hint
Separate a base-width change from a combined-centre-height change.
Compare my reasoning
- The broader base places its tipping edges farther from the initial weight line.
- Carrying the same load lower lowers the combined centre of gravity under the stated otherwise-same conditions.
- Both changes improve the corresponding ideal toppling margin.
A broader base and carrying the same load lower each improve ideal toppling resistance under their stated controls.
Look for these in your work
- Keep each comparison's controls explicit.
- Refer to the combined centre for the loaded vehicle.
Object A has a lower centre of gravity but a much narrower base than B. A student claims A must be more stable. Assess the claim using the qualitative syllabus ideas.
Give me a hint
Ask whether a single-feature comparison is enough when both features change.
Compare my reasoning
- Lower centre height favours stability at a fixed base.
- Narrowing the base reduces the edge margin at a fixed centre position.
- With both changed, the qualitative trends alone do not establish that A must be more stable.
Not established; lower centre and narrower base have opposing effects, so a combined comparison is needed.
Look for these in your work
- Name both relevant changes.
- Avoid an unsupported automatic ranking.
Common mistakes
- State the fixed comparison features.
- Use combined centre of gravity when adding a load.
- Neither a low centre nor a broad base guarantees stability under every disturbance.
What can you explain now?
At the same centre height, what does broadening the base usually do to ideal toppling resistance?
Compare with the explanation
It increases the tipping margin.
The weight line has farther to move before reaching a base edge.
After trying it yourself, choose your next review. This is your self-assessment.
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Make it stick
Explain the separate benefits of lower centre and broader base, then give a case where neither slogan determines the ranking.
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.