Key relationship
ν=−εlateral/εlongitudinal; volume ratio=(1+ε)(1−νε)².
Physics · Class 11
Watch sideways contraction
Mechanical Properties of Solids
An ordinary stretched wire contracts sideways. Poisson ratio compares this negative lateral strain with the positive longitudinal strain. At small strain, the volume change is approximately (1−2ν)ε.
Open the interactive experiment →Watch sideways contraction
இழுக்கப்படும் கம்பி பக்கவாட்டில் சுருங்கும். பாய்சன் விகிதம் பக்கத் திரிபின் எதிர்மறையை நீளத் திரிபால் வகுத்த மதிப்பு.
Key notes, relationships and model limits
Remember
An ordinary stretched wire contracts sideways. Poisson ratio compares this negative lateral strain with the positive longitudinal strain. At small strain, the volume change is approximately (1−2ν)ε.
Common trap
Poisson ratio is a ratio of strains, not a ratio of forces.
Model limits
Small tensile strain in a uniform isotropic material. Geometric volume is evaluated with the displayed longitudinal and lateral scale factors; no plasticity.
What you will learn
Relate axial extension, lateral contraction and volume change.
Try the experiment
Try both scenarios, change one control, and predict the result before checking the graph and readouts.
Think of it this way
Change one input while keeping the others fixed, as you would in a controlled experiment.
Worked example
A 1 m specimen extends 2 mm at ε=0.002. A 10 mm diameter contracts by 0.006 mm when ν=0.3.
Why this matters
The model connects watch sideways contraction with an observable reading. Check the equation and assumptions before applying it.
Before you start
SI units · algebra · graph reading
Check your understanding
At ν=0.3 and longitudinal strain 0.002, what is lateral strain?
- +0.0006
- −0.0006
- −0.006
Read the explanation
εlateral=−νε=−0.3×0.002.
Course connection: Class XI Chapter 8. Consult the roadmap for CBSE, NEET, JEE Main and Advanced scope.