Q.A spring balance shows wrong readings after using for a long time. Why?

The elastic properties of materials, and especially how much they bend under load, are central to structural and mechanical engineering, not just an abstract classroom idea. The elastic behaviour of a beam -- how much it bends for a given load -- is a major factor in the stability of buildings and bridges; a bridge must be designed to withstand the load of flowing traffic, the force of wind, and even its own weight, and 'the amount of stress the design can safely withstand' is a primary safety factor engineers must calculate before construction. To REDUCE the bending of a beam for a given load, a designer should choose a material with a HIGHER Young's modulus -- since Table 7.1 shows steel's Young's modulus (20.0 x $10^{10}$ N m$^{-2}$) is higher than that of aluminium, copper, or glass, with iron close behind (19.0 x $10^{10}$ N m$^{-2}$), steel is the material of choice for heavy-duty machines and iron rods for reinforcing concrete in building construction. A closely related everyday fact -- that steel is more elastic than rubber, even though rubber stretches far more easily -- follows directly from the definition of Young's modulus: for the SAME applied stress, steel produces MUCH LESS strain than rubber does, so steel's Young's modulus (and hence its 'elasticity' in the strict physics sense: the material with the HIGHER Young's modulus i …