The velocity of longitudinal waves will be highest in

• A. Aluminum
• B. Water
• C. Plastic
• D. Air

Answer: (A)

The speed of longitudinal (compressional) waves is mainly governed by how stiff the material is and, to a lesser extent, how dense it is. Stiff materials resist compression and pass the disturbance along faster, while very compressible or dense materials slow it down.

Aluminum is a solid with a high elastic stiffness (great resistance to deformation) and a moderate density. This combination makes it capable of transmitting compressional waves much faster than the other options. Water is denser but not as stiff, so its waves move slower than in metals. Plastics are typically less stiff than metals, so their speeds are intermediate. Air is highly compressible and very low in stiffness, so waves travel the slowest there.

So, the fastest longitudinal-wave velocity among these is in aluminum.