Physics · Mechanical Properties Of Solids · NEET
No. They are two different points on the stress-strain curve, though they are very close for most metals. The proportional limit (point A in NCERT) is where the straight line ends. The elastic limit (point B) is a bit higher, where the body can still spring back to its original size. So proportional limit is reached first, at a slightly lower stress.
The proportional limit comes first (lower stress). Order along the curve: O (origin) to A = proportional limit, A to B = still elastic but curved, B = elastic limit / yield point, then plastic region. Between A and B stress and strain are no longer proportional, but the body still returns to its original shape.
No, and this is the exact trap. Hooke's law (stress proportional to strain) holds only up to the proportional limit, not the elastic limit. Just after the proportional limit the graph curves, so stress is no longer proportional to strain even though the material is still elastic up to the elastic limit.
Yes. In the small region from A (proportional limit) to B (elastic limit) the body still returns fully to its original dimension when the load is removed, so it is elastic. It just does not obey the simple straight-line stress = Y x strain relation there.
In the NCERT curve the yield point (point B) is treated as the elastic limit, and the stress there is the yield strength. Beyond this point permanent (plastic) deformation begins, so the body no longer returns to its original shape.
The maximum elongation of a steel wire of 1 m length if the elastic limit of steel and its Young's modulus respectively are 8 x 10^8 N m^-2 and 2 x 10^11 N m^-2, is
Try the real previous-year questions from this chapter — each with the answer and a full solution.
The proportional limit is the highest stress up to which stress stays directly proportional to strain, so the stress-strain graph is a straight line and Hooke's law holds.
The elastic limit is the maximum stress a body can bear and still return completely to its original shape once the load is removed.
Because the straight-line (proportional) part of the curve ends before the body loses its ability to spring back. There is a short curved but still-elastic region between the two, so the proportional limit is at a smaller stress.
The material enters the plastic region. A permanent set is left behind, meaning that even when the stress becomes zero the strain is not zero, so the body does not regain its original length.
NEET mostly tests the elastic limit through numericals (like the 2024 wire problem) and tests the difference conceptually. Remember that Hooke's law is tied to the proportional limit, while returning to original shape is tied to the elastic limit.
