Arrange the following media from fastest to slowest propagation speed in a typical medium:

• A. Gas, Solid, Liquid
• B. Liquid, Solid, Gas
• C. Solid, Liquid, Gas
• D. Gas, Liquid, Solid

Answer: (C)

The speed of a propagating wave depends on how easily the medium can transfer motion from molecule to molecule, which comes down to stiffness (elastic properties) and density. In a solid, the particles are tightly bound and the material has a high stiffness, so disturbances are transmitted rapidly; despite higher density, the strong restoring forces dominate, giving the highest speeds. In liquids, particles are less tightly bound and can slip past each other, so the medium is more compressible and the restoring forces are weaker, which slows the wave down. In gases, particles are far apart and the compressibility is even greater with very small restoring forces, making the propagation slowest. So the order from fastest to slowest is solid, then liquid, then gas. For example, sound travels much faster in steel than in water, and much faster in water than in air, illustrating this trend.