Explain stretching compression bending and twisting
Recognise that forces can change an object's size or shape as well as its motion.
Exam scope and assessment notes
Core and Extended · Physics 0625, 2026–2028, section 1.5.1. Core covers deformation, load–extension experiments and graph interpretation, collinear resultants, constant velocity, friction and drag. Numerical spring constants, the limit of proportionality, F = ma and the specified circular-motion comparisons are Supplement work in separately labelled Extended lessons. Written practice is self-checked; cumulative assessment, supervised practical experience and human subject-teacher review remain pending.
Before you begin
- Recognise a force as a push or pull.
- Compare an object's original and changed dimensions.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
A spring changes from 12 cm to 14 cm. What is the increase?
What you will learn
- Describe deformation produced by forces.
- Distinguish a shape change from a change of velocity.
Observe the object, not only its position
Pulling a spring can increase its length, squeezing a foam block can reduce its length, and opposed forces can bend a strip or twist a rod. These are changes of size or shape. Forces need not make the whole object travel across the room to have an observable effect.
Two balanced outward forces on a spring can stretch it while the spring's centre stays at rest. A zero resultant on the whole object therefore does not mean there are no forces or no deformation. Which part of the object is loaded and how it is supported affect the shape change.
Describe what changes and what you have measured
State the initial and final size or shape. If a spring's length rises from 12.0 cm to 14.0 cm, its extension is the increase, 2.0 cm. The extension is not the whole 14.0 cm length. A compressed block's reduction in length is also found by comparing the two readings.
A force may change an object's velocity, deform it, or do both. An object sliding without changing shape is not a counterexample to deformation by other loading arrangements. Nor should you claim every material returns to its original shape after a force is removed; that depends on the material and loading.
Pause and explain
A spring is stretched by two balanced outward pulls. Must its extension be zero?
Worked example
A supported spring grows from 12.0 cm to 14.0 cm while its centre does not travel. Identify the force effect and extension.
Show the worked solution
- The changed length is a deformation.
- Extension = 14.0 − 12.0 = 2.0 cm.
- The centre's rest does not exclude forces on the spring.
Answer Stretching; extension 2.0 cm.
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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 changes from 12.0 cm to 14.0 cm. Name its deformation and calculate extension.
Give me a hint
Compare the final length with the unloaded length.
Compare my reasoning
- The spring becomes longer, so it stretches.
- 14.0 − 12.0 = 2.0 cm.
- The final length and extension are different quantities.
Stretching; 2.0 cm extension.
Look for these in your work
- Name the shape change.
- Subtract the original reading.
A foam block's length changes from 8.0 cm to 6.5 cm under opposed pushes. Describe the effect and reduction.
Give me a hint
Opposed pushes can reduce the measured length.
Compare my reasoning
- The block is compressed.
- Reduction = 8.0 − 6.5 = 1.5 cm.
- That decrease describes deformation, not distance travelled.
Compression; length decreases by 1.5 cm.
Look for these in your work
- Distinguish deformation from motion.
- Calculate the change.
A stationary ruler bends when pressed between supports. A student says forces did nothing because its centre did not move along the bench. Correct the claim.
Give me a hint
Look for a change of shape as well as translation.
Compare my reasoning
- The ruler's shape changed.
- Forces produced bending even without whole-object travel.
- Centre motion alone is insufficient to identify all force effects.
Forces bent the ruler; absence of whole-object travel does not mean no effect.
Look for these in your work
- Identify bending.
- Explain the mistaken motion-only criterion.
Common mistakes
- Extension is the change in length.
- Balanced forces do not imply no deformation.
- Not every shape change is reversible.
What can you explain now?
Can forces deform an object whose centre stays at rest?
Compare with the explanation
Yes; forces can change size or shape without whole-object translation.
A supported object can have balanced overall forces and still be stretched or compressed.
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Make it stick
Give a stretching, compression and bending example, naming the change you would observe.
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.