Physics · Mechanical Properties Of Solids · NEET
The theoretical range from elasticity theory is -1 to +0.5. But for almost every real material used in NEET problems, the practical range is 0 to +0.5. Common solids like metals sit around 0.2 to 0.4. So if an option shows a value like 0.3, it is physically reasonable; a value like 0.8 or 2 is impossible.
At sigma = 0.5 the material keeps its volume constant when stretched (the sideways shrinking exactly cancels the lengthwise stretch, so total volume does not change). If sigma were more than 0.5, the material would have to gain volume when compressed, which is impossible for a stable solid. So 0.5 is the upper limit, reached (nearly) by rubber-like materials.
Yes, in theory it can be as low as -1, and a few special materials called auxetics (some foams) get thicker when you stretch them, giving a negative sigma. But normal materials always get thinner when stretched, so sigma is positive for every material you meet in NEET. Treat negative sigma as a rare exception, not the default.
It means the material is incompressible: its volume stays the same during stretching. The lateral (sideways) strain is exactly half the longitudinal strain, so the loss of width perfectly balances the gain in length. Rubber is close to this value, which is why rubber is treated as almost volume-preserving.
Steel is about 0.28 to 0.30, copper about 0.33, aluminium about 0.33, glass about 0.20 to 0.25, rubber close to 0.5, and cork close to 0 (cork barely changes width when compressed, which is why it fits into a bottle). Knowing rubber near 0.5 and cork near 0 helps you eliminate wrong options fast.
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Yes. It is the ratio of lateral strain to longitudinal strain, and both strains are pure ratios (change divided by original), so Poisson's ratio has no unit and no dimension.
For an ideal incompressible material sigma = 0.5. Many textbooks quote about 0.3 as a typical average for common metals, so 0.3 is a safe default if a value is not given.
Rubber changes shape easily but resists volume change, so its lateral shrinking almost exactly balances its stretch, keeping volume nearly constant. That behaviour gives a Poisson's ratio near the upper limit of 0.5.
No. Poisson's ratio only describes how much a material narrows when stretched; it does not measure strength or stiffness. Strength links to breaking stress and stiffness links to Young's modulus, which are separate quantities.
Yes, for a material that does not change width at all when stretched, like cork. Cork's near-zero Poisson's ratio is exactly why it is used as a bottle stopper: pushing it does not make it bulge sideways.
