In which medium would have the highest velocity of longitudinal waves among Aluminum, Water, Air, Plastic?

In which medium would have the highest velocity of longitudinal waves among Aluminum, Water, Air, Plastic?

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

Answer: (A)

The speed of a longitudinal wave is mainly set by how stiff (elastic) the material is and how dense it is. A medium that is very stiff and not too dense lets compressions travel faster. Air is very compressible and light, so longitudinal waves travel slowly. Water is denser and less compressible, giving a higher speed than air but still well below solids. Plastics vary a lot, but many have moderate stiffness and density, yielding speeds in the lower solid range. Aluminum, with high stiffness (elastic modulus) and a reasonable density, transmits compressions much faster, giving the highest longitudinal-wave velocity among these options. So aluminum has the fastest longitudinal-wave speed.

The speed of a longitudinal wave is mainly set by how stiff (elastic) the material is and how dense it is. A medium that is very stiff and not too dense lets compressions travel faster.

Air is very compressible and light, so longitudinal waves travel slowly. Water is denser and less compressible, giving a higher speed than air but still well below solids. Plastics vary a lot, but many have moderate stiffness and density, yielding speeds in the lower solid range. Aluminum, with high stiffness (elastic modulus) and a reasonable density, transmits compressions much faster, giving the highest longitudinal-wave velocity among these options.

So aluminum has the fastest longitudinal-wave speed.