In a longitudinal wave, how does the direction of vibration of the particles compare to the direction of motion of the wave?

• A. The direction of vibration is at a $45°$ angle to the direction of wave motion.
• B. The direction of vibration is parallel to the direction of wave motion.
• C. The direction of vibration is perpendicular to the direction of wave motion.
• D. The direction of vibration is opposite to the direction of wave motion.

Answer: (B)

Verified step by step guidance

1. Understand the nature of a longitudinal wave: In a longitudinal wave, the particles of the medium vibrate back and forth along the same line as the wave travels.
2. Recall the definition of particle vibration direction: The vibration direction refers to how the particles oscillate around their equilibrium position.
3. Recall the direction of wave motion: The wave motion direction is the direction in which the wave energy propagates through the medium.
4. Compare the two directions: Since in a longitudinal wave the particles move parallel to the wave propagation, the direction of vibration is parallel to the direction of wave motion.
5. Conclude that the vibration direction is neither at an angle, perpendicular, nor opposite, but parallel to the wave motion direction.