The angle between the particle velocity and wave velocity in a transverse wave is [except when the particle passes through the mean position]

A motor-cyclist moving towards a huge cliff with a speed of $18 \mathrm{kmh}^{-1}$, blows a horn of source frequency 325 Hz . If the speed of the sound in air is $330 \mathrm{~ms}^{-1}$, the number of beats heard by him is

A galaxy is moving away from the Earth so that a spectral line at 600 nm is observed at 601 nm . Then, the speed of the galaxy with respect to the Earth is

To propagate both longitudinal and transverse waves, a material must have