Explain stability using the weight line and tipping edge
Use a restoring or toppling moment instead of claiming that every small tilt makes an object fall.
Section 1.5.3 · Centre of gravity
Exam scope and assessment notes
Core and Extended · Physics 0625, 2026–2028, section 1.5.3. Define centre of gravity, describe its determination for an irregular plane lamina, and explain qualitatively how its position affects stability. Compare the weight line with the base or tipping edge; no critical-angle calculation is required here. Written practice is self-checked. Supervised practical experience, cumulative assessment and human subject-teacher review remain pending.
Before you begin
- Identify the vertical line through centre of gravity.
- Describe moment direction about a pivot.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
Which line must be compared with a tipping edge?
What you will learn
- Explain restoring and toppling tendencies qualitatively.
- Relate the vertical weight line to a base edge during a tilt.
Track weight as the object tilts
For a simple rigid object resting on a level base, its vertical weight line should lie within the supported base for ordinary static balance. As the object tilts about a base edge, consider the moment of weight about that edge. If the centre of gravity remains on the returning side of the edge, its weight tends to restore the object to its base after a small release.
If the weight line moves to the outside of the tipping edge, weight instead tends to turn the object farther over. At the limiting position where the line passes through the edge, weight has zero moment about it; this is a boundary rather than a guarantee of robust stability. Disturbances or another applied force can determine what happens at or near that boundary.
Use the line of action, not the nearest visible corner
The original model diagram follows the same rectangular body through two clockwise tilts about its right base edge. In the first position the vertical weight line lies left of that edge and gives an anticlockwise restoring tendency. In the second it lies right and gives a clockwise toppling tendency. The arrows are schematic weight-direction indicators rather than a measured prediction of the subsequent motion.
This qualitative explanation assumes a rigid body, suitable contact, a fixed tipping edge and no other unaccounted turning forces after release. Sliding can occur instead if contact cannot provide the needed friction, and an external push can temporarily supply another moment. State the ideal tipping assumptions rather than promising that every real object follows only the weight moment.
Pause and explain
For a right-edge pivot, a downward weight line lies to its right. What does weight tend to do?
Worked example
A body is tilted about its right base edge. Its centre of gravity's vertical line lies left of that edge. What is the weight's turning tendency?
Show the worked solution
- Choose the right edge as the pivot.
- The downward weight line is to the left of the edge.
- Its moment is anticlockwise, tending to restore the body rather than topple it farther right.
Answer Anticlockwise restoring tendency about the right edge.
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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.
For a right-edge pivot with the weight line left of it, identify the weight moment and release tendency in the ideal tipping model.
Give me a hint
Imagine a downward force on the left of the chosen pivot.
Compare my reasoning
- Weight acts downward through centre of gravity.
- A downward force left of the edge has an anticlockwise moment.
- That tendency returns the body towards its resting base.
Anticlockwise restoring tendency.
Look for these in your work
- Use the correct edge as pivot.
- Connect the moment direction to restoration.
Compare weight lines left of, through, and right of a right tipping edge.
Give me a hint
The signed lever arm changes as the force line crosses the edge.
Compare my reasoning
- Left gives an anticlockwise restoring tendency.
- Through the edge gives zero weight moment and the limiting position.
- Right gives a clockwise toppling tendency.
Left: restoring; through: zero weight moment at the limit; right: toppling.
Look for these in your work
- Distinguish all three positions.
- Avoid calling the boundary a robust stable state.
A tilted box slides before it topples. Explain why a diagram of the weight moment alone did not guarantee its actual motion.
Give me a hint
Check the assumed contact and fixed-edge model.
Compare my reasoning
- The tipping explanation assumed suitable contact and a fixed pivot edge.
- Insufficient friction allows a different sliding motion.
- Actual motion requires the contact forces and any applied push as well as weight to be considered.
The fixed-edge tipping assumptions failed; contact forces and friction permit sliding instead.
Look for these in your work
- Identify the failed assumption.
- Include forces beyond the weight-only moment argument.
Common mistakes
- Track the vertical weight line through centre of gravity.
- An edge-aligned line is a limiting position rather than a stable margin.
- State contact and no-other-force assumptions.
What can you explain now?
What changes when the weight line moves outside the right tipping edge?
Compare with the explanation
Weight gives a clockwise toppling tendency.
The force line has moved to the opposite side of the same pivot.
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Make it stick
Draw a weight line on either side of one tipping edge and explain why its moment reverses.
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.