NCERT says steel is more elastic than rubber, but rubber stretches so much and returns. What am I missing?

7 answers & replies

• [+][–]Dev Agarwal12thSample· 1mo ago

  I think rubber is more elastic because it deforms a lot and still returns. Young's modulus is just a number NCERT uses to compare things, it's not the real definition of elasticity.

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• [+][–]Arjun Nair12thSample· 1mo ago

  Dev, careful. In physics, how elastic a material is gets compared through the modulus, i.e. how much stress you need for a given strain. NCERT's values: steel $Y \approx 2.0\times10^{11}$ N m⁻², copper about $1.1\times10^{11}$, aluminium about $0.7\times10^{11}$. Rubber's modulus is a very tiny fraction of that.

  Take the same stress on both:

  $\text{strain} = \frac{\text{stress}}{Y}$

  Steel strains very little, rubber a lot. So steel resists deformation much more and develops a much bigger restoring stress for the same strain. The word "more elastic" in NCERT means greater tendency to resist deformation and restore. Not "stretches more".

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  • [+][–]Ananya Reddy11thSample· 1mo agoAuthor

    this is the line I needed: "stretches more ≠ more elastic". rewriting my pink box now. also adding Y values for steel, copper, aluminium in my green box ✅

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  • [+][–]Dev Agarwal12thSample· 1mo ago

    yeah got it, I was mixing the everyday word elastic with the physics one. my bad

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• [+][–]Ishita GoyalDropperSample· 1mo ago

  Spring–ball model, answering a, b, c:

  a) Each atom sits at an equilibrium separation where the net inter-atomic force is zero. Stretch it and the attractive force pulls it back. Remove the load and it returns.

  b) Y depends on how stiff those "springs" are. In steel the bonds between atoms are strongly resistive, so a given stretch needs a lot of force. Y is large.

  c) Beyond the elastic limit, the layers of atoms slip past each other and settle at new equilibrium positions. The bonds re-form in the new place, so when you remove the load there is nothing pulling it back to the old shape. That's permanent (plastic) deformation.

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• [+][–]Aman YadavDropperSample· 1mo ago

  Elastic vs perfectly elastic. An elastic body regains its shape after the force is removed, up to the elastic limit. A perfectly elastic body regains its original shape completely, however long it was loaded. No real body is perfect. Quartz is considered the closest to it. At the other end, putty is the closest to perfectly plastic: it keeps whatever new shape you give it.

  Exam note: "elastic limit" = max stress up to which the body returns to the original shape.

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• [+][–]Meera Patel11thSample· 1mo ago

  A small addition about rubber specifically. The stress–strain curve of rubber is not a straight line, it doesn't follow Hooke's law in the way steel does, and there is no clear plastic region. It can be stretched to several times its length and still return, but the loading and unloading curves don't overlap exactly; this is called elastic hysteresis, and the area between them is the energy lost as heat in each cycle. So you can't even compare it fairly using a single value of Y like you do for a metal wire. That's the reason NCERT labels such materials "elastomers" and treats them separately.

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