What is meant by compressive stress?

The two blocks $P$ and $Q$ are connected by a metal wire with breaking stress ${10}^9 \mathrm{N} {\mathrm{m}}^{-2}$ through a frictionless pulley. The minimum radius of the wire for it not to break is (consider $g=10 \mathrm{m} {\mathrm{s}}^{-2}$)

A copper and a steel wire of the same diameter are connected end to end. A deforming force $F$ is applied to this composite wire, which causes a total elongation of $1 \mathrm{cm}$. The two wires will have, 

A block of mass $\mathrm{4 }\text{kg}$ is suspended from the ceiling with steel wire of radius $\mathrm{2 }\text{mm}$ and negligible mass. Find stress in the wire (Given$g={\pi }^2$)

A rod of length $l$ and negligible mass is suspended at its two ends by two wires of steel (wire $A$) and aluminium (wire $B$) of equal lengths (see figure).
The cross-sectional areas of wires $A$ and $B$ are $1.0{\mathrm{mm}}^2$ and $2.0{\mathrm{mm}}^2$, respectively.

$\left(Y_{\mathrm{AL}}=70\times {10}^9{\mathrm{Nm}}^{-2}\mathrm{and}{Y}_{\mathrm{steel}}=200\times {10}^9{\mathrm{Nm}}^{-2}\right)$