9702 · 6.1
Stress and strain — FAQ
Frequently asked questions for 9702 Stress and strain. Direct answers first, then deeper explanation — then practise with marking.
Why is strain a dimensionless quantity?
Strain is calculated as a ratio of two lengths (extension divided by original length). Since the units of length cancel out, strain has no units, making it a pure number that represents a relative change.
What is the key difference between the spring constant ($k$) and the Young Modulus ($E$)?
The spring constant () describes the stiffness of a specific object (like a particular spring or wire), and it depends on that object's dimensions. The Young Modulus () is an intrinsic property of the material itself, independent of its shape or size, used to compare the stiffness of different substances.
Can Hooke's Law be applied during plastic deformation?
No, Hooke's Law states that force is directly proportional to extension and is only valid up to the limit of proportionality, which typically occurs before or at the elastic limit. Beyond this point, the material no longer behaves elastically, and the relationship between force and extension becomes non-linear.