What is the restoring force for Capillary Waves?

• A. Gravity
• B. Buoyancy
• C. Inertia
• D. Surface Tension

Answer: (D)

Capillary waves are driven by surface tension acting as the restoring force. When the surface is curved by a ripple, surface tension pulls along the interface to minimize its area, pulling crests down and troughs up until the surface tends to flatten again. For the small wavelengths that define capillary waves, gravity is comparatively weak, so it does not significantly restore the surface; instead, the tension at the surface does. Inertia supplies the motion, but it isn’t the restoring force, and buoyancy isn’t the restoring force for this type of surface wave.