Describe tensile and compressive deformation
Distinguish stretching from compression and use load, extension and the limit of proportionality to describe a solid's response.
AS Level; Cambridge International Physics 9702, 2025–2027 syllabus, Topic 6. The wire investigation is an original study guide and data-analysis practice, not a supervised laboratory activity or proof of practical competence. Full timed assessment and human subject-teacher review remain outstanding.
Before you begin
- Use force, area, length, extension and graph gradient with consistent SI units.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
Which load tends to increase the length of a sample?
What you will learn
- Relate deformation to tensile and compressive forces in one dimension
- Use load, extension and compression correctly
- Identify the limit of proportionality on a force-extension relationship
Tension and compression change a solid's dimensions
A tensile force pulls a solid and tends to increase its length. A compressive force pushes and tends to shorten it. The syllabus model treats the force and deformation in one dimension.
Load is the applied force. Extension is the increase in length from the original length; compression is the decrease. Use a consistent reference length so the sign and size of the dimensional change are clear.
Proportional response has a boundary
For a sample under tensile load, plot load or force against extension. At small loads the graph may be a straight line through the origin, showing extension proportional to load.
The limit of proportionality is the point beyond which force is no longer proportional to extension. It is not automatically the same as the elastic limit: a material may remain elastic after the straight-line proportional region ends.
Pause and explain
What marks the limit of proportionality?
Worked example
A wire has original length 1.20 m and length 1.203 m under a tensile load. Find its extension in millimetres and identify the deformation type.
Show the worked solution
- Extension = loaded length − original length.
- Extension = 1.203 − 1.200 = 0.003 m.
- Convert 0.003 m to 3.0 mm; tensile loading stretches the wire.
Answer Extension 3.0 mm; tensile deformation.
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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.
A spring's length increases from 0.150 m to 0.158 m under a tensile load. Find its extension.
Give me a hint
Subtract original length from loaded length.
Compare my reasoning
- Extension = 0.158 − 0.150 m.
- Extension = 0.008 m.
- Convert to millimetres if useful.
0.008 m = 8.0 mm extension.
Look for these in your work
- Original length subtracted
- Difference calculated
- Extension unit stated
A rod's length decreases from 0.80 m to 0.77 m under a compressive force. Find the compression.
Give me a hint
Compression is original length minus the shorter loaded length.
Compare my reasoning
- Compression = 0.80 − 0.77 m.
- Compression = 0.03 m.
- State the deformation is compression.
0.030 m, or 3.0 cm, compression.
Look for these in your work
- Correct subtraction order
- Magnitude calculated
- Compression named
A force-extension graph is straight through the origin up to 4.0 mm, then curves but returns to the original length when unloaded at 6.0 mm. State the limit of proportionality and explain why it is not the elastic limit in this example.
Give me a hint
Separate proportional response from whether deformation is recovered after unloading.
Compare my reasoning
- Proportional extension ends where the graph stops being straight: at 4.0 mm.
- The sample still returns to its original length after unloading at 6.0 mm.
- So the proportionality limit is 4.0 mm, while the elastic limit is beyond 6.0 mm for the observations given.
Limit of proportionality: 4.0 mm. It is not the elastic limit here because the sample still recovers its original length after unloading at 6.0 mm.
Look for these in your work
- Graph boundary identified
- Recovery evidence used
- The two limits distinguished
Common mistakes
- Confusing extension with final length
- Calling a shorter length an extension
- Treating limit of proportionality and elastic limit as identical
What can you explain now?
Which load tends to increase the length of a sample?
Compare with the explanation
Tensile load
Tension pulls and stretches the sample.
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Make it stick
How can a material be outside its proportional region but still deform elastically?
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