Which one of the following is the weakest kind of bonding in solids?

If a string,suspended from the ceiling,is given a downward force $F_1$,its length becomes $L_1$. Its length becomes $L_2$ if the downward force is $F_2$. What is its actual length?

Why does strain have no unit?

$A$ wire can be broken by applying a load of $20 \, kg$ weight. The force required to break the wire of twice the diameter is .......... $kg \, wt$.

Given below are two statements: one is labelled as Assertion $A$ and the other is labelled as Reason $R$.
Assertion $A$: Steel is used in the construction of buildings and bridges.
Reason $R$: Steel is more elastic and its elastic limit is high.
In the light of the above statements,choose the most appropriate answer from the options given below:

Two blocks of masses $3 \, kg$ and $5 \, kg$ are connected by a metal wire going over a smooth pulley. The breaking stress of the metal is $\frac{24}{\pi} \times 10^{2} \, Nm^{-2}$. What is the minimum radius of the wire? (Take $g = 10 \, ms^{-2}$) (in $cm$)