What is a perfectly elastic body and a perfectly plastic body?

Two wires as shown in the figure below,made of steel,have a breaking stress of $12 \times 10^8 \text{ N/m}^2$. The area of cross-section of the upper wire is $0.008 \text{ cm}^2$ and that of the lower wire is $0.004 \text{ cm}^2$. The maximum mass that can be added to the pan without breaking any wire is . . . . . . kg. (Take $g = 10 \text{ m/s}^2$)

The length of a rod under longitudinal tension $T_1$ is $L_1$ and that under longitudinal tension $T_2$ is $L_2$. What is the actual length of the rod,in the absence of tensions?

The two thigh bones (femurs),each of cross-sectional area $10 \, cm^2$,support the upper part of a person of mass $50 \, kg$. The average pressure sustained by the femurs is ............. $N/m^2$.

$A$ wire of diameter $1\, mm$ breaks under a tension of $1000\, N.$ Another wire,of the same material as that of the first one,but of diameter $2\, mm$ breaks under a tension of ...... $N$.

The length of a metal wire is $l_1$ when the tension in it is $T_1$,and it is $l_2$ when the tension is $T_2$. The unstretched length of the wire is