Question

A spring balance deigned for a maximum load of 400 N was used to weigh a load of 500 N. After the 500 N load had been removed, the pointer would not return to the zero mark on the scale. Why?

Answer 1

It happens, because the stress 500N exceeded the maximum designed value 400N. Thus, the spring reached the yield point - the poin on the stress-strain curve that indicates the limit of reversible deformation (see https://en.wikipedia.org/wiki/Yield_(engineering)). Above this point a material becames plastic and some extra strain remains even when load is removed.