The elasticity of Invar:

If the length of a string is $P$ when the tension in it is $6 \ N$ and its length is $Q$ when the tension in it is $8 \ N$, then the original length of the string is

In solids,inter-atomic forces are

Match the physical quantities in Column-$I$ with the details given in Column-$II$ appropriately:

Column-$I$	Column-$II$
$(a)$ $\text{Stress} \propto \text{Strain}$	$(i)$ $M^1 L^{-1} T^{-2}$
$(b)$ $\text{Dimensional formula for compressibility}$	$(ii)$ $M^{-1} L^1 T^2$
	$(iii)$ $\text{Poisson's ratio}$
	$(iv)$ $\text{Hooke's law}$

When is the force between atoms attractive? When is it repulsive?

One end of a wire of length $L$ and weight $W$ is attached to a ceiling,and the other end is attached to a weight $W_1$. If the cross-sectional area of the wire is $S$,what is the stress at a distance $3L/4$ from the lower end of the wire?