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MCAT Physics Question — Velocity of Sound

Which of the following statements accurately orders the speed of sound through the following mediums: air (ρair = 1.275 kg/m3), water (ρwater = 1000 kg/m3), and copper (ρcopper = 8960 kg/m3)?


A. Air > water > copper
B. Air > copper > water
C. Copper > water > air
D. Copper > air > water

Click for Explanation
This question asks the examinee to identify the correct order for the velocity of sound waves through various media. The velocity of sound is given by the equation, v = (B/ρ)1/2, where B is the bulk modulus and ρ is the density of the material.

The densities of solids are the greatest because of the strong intermolecular forces, which create lattice structures with closely bonded molecules. While solids tend to be densest, they also tend to be the least compressible, which corresponds to a high bulk’s modulus. As a result, the B/ρ quotient is the highest for solids, followed by liquids, and then gases. Therefore, the velocity of sound through copper is greater than water, which is greater than air. Thus, C is the correct answer.

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