In a liquid medium, which mode of vibration can exist?

• A. Surface
• B. Longitudinal
• C. Both A and B
• D. Shear

Answer: (B)

Liquids can’t sustain shear (transverse) stresses, so shear waves don’t propagate in a liquid. The medium can, however, support compressional or longitudinal waves, where particles oscillate in the same direction as the wave travels and the fluid is alternately compressed and rarefied as the wave passes. The speed of these waves in a liquid depends on its bulk modulus and density (v = sqrt(K/ρ)). Surface waves can exist at interfaces, but they are not bulk modes inside the liquid. So the mode that can exist in a liquid medium is longitudinal (compressional) waves.