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In the elastic region where stress is proportional to strain up to elastic limit which is we call Young's Modulus (E). My question is if that is E, then what is Hooke's Law (K=F/x spring stiffness). I thought that Hooke's Law is applicable in the elastic limit too. How can we say that both are different? Its a bit confusing. Can we have a simple answer? Pictures are attached. Thank you.

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    $\begingroup$ See engineering.stackexchange.com/a/48995/10902 $\endgroup$
    – Solar Mike
    Mar 17 at 16:12
  • $\begingroup$ I don't think he would've asked again if he had already received a sufficient answer to aide him when he previously asked. $\endgroup$
    – Farris
    Mar 17 at 18:56
  • $\begingroup$ It was not clear answer. $\endgroup$ Mar 18 at 8:40

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Hooke's law was initially stated by Robert Hooke in a paper on 1676, stating that the force required to extend a spring with stiffness of K was proportional to the distance, X

$$F=KX \rightarrow \ K=\frac{F}{X}$$

But later the concept was extended to most solid bodies acting within their elastic range.

For example, in a rod, the Hooke's stiffness K and the Young modulus are related by:

$$K=E \ \frac{A}{L}\quad and\quad E= K\ \frac{L}{A} $$

  • E= Young modulus
  • A= area of the rod
  • L= length of the rod.
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  • $\begingroup$ What is the difference between these two? E (Young's Modulus) and stiffness constant (K) both exist in elastic range. I am still confused. But thanks. $\endgroup$ Mar 18 at 8:43
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    $\begingroup$ @IbrahimOmer, maybe this will help . if we double the length of the rod in my answer its K becomes half, but its young modulus remains the same. K is related to the geometey but E is a material property. $\endgroup$
    – kamran
    Mar 18 at 17:02
  • $\begingroup$ Then what is the difference between elasticity and stiffness as both lie in elastic range? $\endgroup$ Mar 18 at 20:02
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Hooke's law is a law of physics that states that the force (F) needed to extend or compress a spring by some distance (x) scales linearly with respect to that distance—that is, $F_s = kx$.

Young's modulus is a measure of a solid's stiffness or resistance to elastic deformation under load.

ADD: - Another way to see the relationship between the Hookie's Law and Young's Modulus (E), and Young's Modulus (E) vs the Spring Constant (k)

Hooke's Law describes linear-elastic behavior and is mathematically expressed as $\Delta = \dfrac{PL}{EA}$. And, since $\delta = \dfrac{P}{A}$, it can be expressed as $\Delta = \delta\dfrac{L}{E}$.

Let's replace $\dfrac{P}{\Delta}$ with $k$ and rearrange the first expression. It becomes

  • $E = \dfrac{PL}{A\Delta} = k\dfrac{L}{A}$ - note that in this case, $E$ is directly related with $k$, thus both can be considered and called the "stiffness" of an element.

Now rearrange the second expression as,

$\dfrac{\Delta}{L} = \dfrac{\delta}{E} = \epsilon$, from this

  • $E = \dfrac{\delta}{\epsilon}$ - in here, $E$ is a measurement of the ratio of stress vs strain of an element subjected to force.

Conclusion - Both "$E$" and "$k$" are a component in expressing Hooke's Law. The difference is the thing they measure - $E$ measures stress vs strain ratio; $k$ measures the force-displacement characteristic of an element. As both measurements indicate the level of resistance of an element subjecting to force, either explicitly (as in $k$) or implicitly (as for $E$), both can be used to indicate the "stiffness" of the element.

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  • $\begingroup$ Solid's stiffness or resistance to elastic deformation is K (spring constant). I am still confused. But thanks. $\endgroup$ Mar 18 at 8:40
  • $\begingroup$ Do not overthink. K is related to stiffness in one dimension only (axial force-displacement), while 'E" (stress-strain) is the general indication of material stiffness in resisting stress of all kinds. $\endgroup$
    – r13
    Mar 18 at 15:12
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The main difference between the Young's modulus (E) and the Spring's constant (K) is that Young's modulus refers only to the material, while spring stiffness refers to the structure.

The spring constant is dependent upon the Young's modulus. E.g. for a beam with rectangular cross-section (A) and length (L), K is calculated by:

$$K = \frac{A\cdot E}{L}$$

So for example if the beam kept the same materials and either the length L or the cross-section changed, then the Young's modulus would remain the same and the spring stiffness would change.


Essentially, Hooke discovered was a polymath that was mostly interested in solving problems. The spring constant was part of a series of experiments that Hooke conducted in order to invent a spring watch/a spring balance. Hooke observed that for a structure (in most cases) doubling the force resulted in doubling the deformation, but did not want to "waste" time decoupling the effects of geometry, and materials, so settled for the Spring's constant. And it took over 100 years for the widespread adoption of Young's modulus (although Leonard Euler had already proposed the concept).

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  • $\begingroup$ What if we say in stress strain diagrams during elastic range Hooke's Law is applicable or not? Because there is Young's Modulus E (stress directly proportional to strain). Which statement would be correct? $\endgroup$ Mar 19 at 19:33
  • $\begingroup$ I am sorry I cannot understand the question in your comment. $\endgroup$
    – NMech
    Mar 19 at 21:48
  • $\begingroup$ In elastic range, stress is proportional to strain (E). Similarly, force is directly proportional to distance (K). Which statement is correct? $\endgroup$ Mar 19 at 22:57
  • $\begingroup$ @IbrahimOmer The statements should be revised to: "In linear elastic range, stress is proportional to strain, which defines the Young Modulus (E = stress/strain). Similarly, force is directly proportional to stiffness, as K = F/delta (linear deformation). The statements are correct because both phenomenons follow Hooke's Law and demonstrate linear elastic behavior." $\endgroup$
    – r13
    Mar 19 at 23:38

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