If the distance between the centres of the Earth and the Moon is $D$ and the mass of the Earth is $81$ times the mass of the Moon,then at what distance from the centre of the Earth will the net gravitational field be zero?

Which one of the following is true for an exothermic reaction $A \rightleftharpoons B$,if $E_f$ and $E_b$ are the activation energies of forward and backward reactions respectively?

Which of the following is a copper-releasing $IUD$?

$Mg$ forms $Mg^{2+}$ but not $Mg^+$. The reason is ........

The time period of a particle in simple harmonic motion is $8 \ s$. At $t=0$,it is at the mean position. The ratio of the distances travelled by it in the first and second seconds is

Two cylinders $A$ and $B$ fitted with pistons contain equal amounts of an ideal diatomic gas at $300 \ K$. The piston of $A$ is free to move while that of $B$ is held fixed. The same amount of heat is given to the gas in each cylinder. If the rise in temperature of the gas in $A$ is $30 \ K$,then the rise in temperature of the gas in $B$ is ..... $K$.