Ans. (a) Upto point A, Hooke's law is obeyed because the graph is straight line from O to A.
(b) Point B, from the graph it is dear that the wire
returns to its original position after being unloaded upto point A only, henceJB is elastic limit.
(c) Elastic region—O to B
Plastic region—B to E
(d) Strain varies directly proportional to load upto
point A and after A, strain increases by greater amount as compared to first case (i.e., O to A) for a given increase in load. Beyond the elastic limit B, the curve does not retrace backwards as the wire is unloaded but returns along dotted line CO'. Point O' corresponds to strain at zero load, which shows there is a permanent strain in the wire.