C. When a sound wave travels through air, the air molecules move vertically up and down following the direction of the wave, rather than moving forward or backward along with the wave. As the wave passes, the molecules oscillate from their original positions but ultimately return to equilibrium without experiencing any net displacement in the direction of the wave.

1. What will happen to the air molecules when a sound wave travels through?
A. The molecules move forward, along with the direction of the wave.
B. The molecules move backwards as the wave goes by it.
C. The molecules move awayfrom its original, following the verticaldirection of the wave.
D. The molecules do not move at all.
Answer: C.
“The medium does not travel with the wave” means that the segment will not have a net
displacement in the same direction of the wave. Think about a segment of a string; the segment
cannot travel “through” to the other end of the string. This works for the air molecules as well.
When a wave passes from left to right, the molecules move up and down, eventually back to
their equilibrium states.