In a longitudinal wave, particles move in the same direction as the wave propagates.

• A. Surface
• B. Longitudinal
• C. Transverse
• D. Torsional

Answer: (B)

In a longitudinal wave, the defining trait is how the medium’s particles move relative to the direction the wave travels. The particles oscillate back and forth along the same line as the wave’s propagation, creating compressions and rarefactions that carry energy forward through the medium. Think of sound traveling through air: air molecules compress and then spread out along the direction the sound waves move, even though the overall wave energy flows forward.

This contrasts with transverse waves, where particle motion is perpendicular to the direction of travel (like waves on a string, where the motion is up and down while the wave moves sideways). Torsional motion involves twisting around the direction of travel, not moving along it, and surface waves involve motion that lies at the interface of two media and can have more complex particle paths. So the statement correctly identifies the long-axis, parallel motion characteristic of longitudinal waves.