A body weighs 45 N on the surface of the earth. The gravitational force on a body due to earth at a height equal to half the radius of earth will be

An air column is of length 17 cm . The ratio of frequencies of $5^{\text {th }}$ overtone if the air column is closed at one end to that open at both ends is (velocity of sound in air $=340 \mathrm{~ms}^{-1}$ )

In Paschen series, wavelength of first line is ' $\lambda_1$ ' and for Brackett series, wavelength of first line is ' $\lambda_2$ ' then ratio $\frac{\lambda_1}{\lambda_2}$ is

A sphere of mass ' $m$ ', moving with velocity ' $3 u$ ' collides head-on with another identical sphere at rest. If ' $e$ ' is coefficient of restitution then what will be the ratio of velocity of the second sphere to that of first sphere after collision?