The force constant of a wire is $k$ and that of another wire is $2k$. When both the wires are stretched through the same distance,then the work done is:

$A$ block of mass $1\,kg$ lies on a horizontal surface in a truck. The coefficient of static friction between the block and the surface is $0.6$. If the acceleration of the truck is $5\,m\,s^{-2}$,the frictional force acting on the block is ........ $N$.

The random variable $X$ takes the values $1, 2, 3, \ldots, m$. If $P(X=n) = \frac{1}{m}$ for each $n$,then the variance of $X$ is

The energy of an electron in the first Bohr's orbit of $H$ atom is $-13.6 \ eV$. The possible energy value$(s)$ of the excited state$(s)$ for an electron in Bohr's orbits of hydrogen is (are): ............. $eV$

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

What is the decreasing order of reactivity of $HX$ in the given reaction: $ROH + HX \rightarrow RX + H_2O$?