$A$ wire elongates by $l \, mm$ when a load $W$ is hung from it. If the wire goes over a pulley and two weights $W$ each are hung at the two ends,the elongation of the wire will be (in $mm$)

An air bubble of radius $1 \ cm$ rises from the bottom portion through a liquid of density $1.5 \ g/cc$ at a constant speed of $0.25 \ cm/s$. If the density of air is neglected,the coefficient of viscosity of the liquid is approximately,(in $Pa \cdot s$):

For any integer $n \geq 1$,the sum $\sum_{k=1}^n k(k+2)$ is equal to

Which one of the following is used in the preparation of cellulose nitrate?

The logically equivalent proposition of $p \Leftrightarrow q$ is

$A$ solution of sodium in liquid ammonia is blue in colour due to