In a liquid medium, which statement is true about vibration modes?

• A. Longitudinal and shear modes both exist
• B. Shear is the only mode
• C. Only longitudinal is possible
• D. Surface waves are required

Answer: (C)

In liquids, only bulk longitudinal (compressional) waves can propagate. This happens because fluids can’t sustain shear stresses; their shear rigidity is effectively zero, so there’s no restoring force to support transverse (shear) motion. The waves that move through a liquid involve particles oscillating in the same direction as the wave travels, driven by changes in pressure and density, and their speed depends on the liquid’s bulk modulus and density. Surface waves are a feature of solid surfaces, not bulk waves in a liquid, so they don’t govern wave propagation in the liquid itself. Therefore, only longitudinal waves are possible in a liquid.