Questions: Wave Motion: Definition and Classification

Question 1

A leaf floats on the surface of a still pond. When a wave passes beneath it, what does the leaf do?

• A. It bobs up and down approximately in place — water particles oscillate without traveling in the direction of the wave
• B. It moves steadily in the direction the wave is traveling, carried by the wave's momentum across the pond
• C. It moves perpendicular to the wave's direction of travel, swept sideways by particle motion
• D. It remains completely stationary, since surface waves do not disturb floating objects

Answer: (A)

This is the central insight of wave motion: energy propagates, matter does not. Each water particle at the surface moves in a roughly circular or up-and-down path near its equilibrium position as the wave passes — the particle does not travel with the disturbance. The leaf bobs up and down but ends up roughly where it started. Option B is the most common misconception: students confuse the direction of wave propagation with the direction of particle motion, imagining that matter flows along with the wave.

Question 2

Sound is a longitudinal wave traveling through air. What does this classification tell us about how air molecules move?

• A. Air molecules oscillate back and forth parallel to the direction the sound travels, creating alternating compressions and rarefactions
• B. Air molecules oscillate perpendicular to the direction the sound travels, like a rope shaken sideways
• C. Air molecules travel from the sound source to the listener, carrying kinetic energy with them
• D. Air molecules do not move at all — only pressure values change propagate through the medium

Answer: (A)

In a longitudinal wave, particle displacement is parallel to the direction of propagation — the defining characteristic. Air molecules get pushed back and forth along the direction the sound is heading, creating regions of high pressure (compressions) and low pressure (rarefactions). This contrasts with transverse waves (like light or a shaken rope), where particle motion is perpendicular to propagation. Option C is the wave-as-matter-flow misconception: molecules don't travel from source to listener; the disturbance pattern does.

Question 3

In a wave, energy is transported through a medium without the medium itself being permanently displaced in the direction of propagation.

• T. True
• F. False

Answer: True

This is the defining property of wave motion. Each particle in the medium oscillates around its equilibrium position as the wave passes, but returns to approximately where it started. The disturbance — the pattern of displacement — moves through the medium, carrying energy with it, while the matter stays put. This is what distinguishes wave propagation from bulk flow (like wind or current), where matter actually moves from one place to another.

Question 4

The speed at which a wave travels through a medium depends primarily on the wave's amplitude — larger disturbances travel faster.

• T. True
• F. False

Answer: False

Wave speed depends on properties of the medium, not on the amplitude or frequency of the wave. For a string, wave speed depends on tension and linear density; for sound in air, it depends on bulk modulus and density; for light in a vacuum, it is the constant c. Amplitude affects how much energy the wave carries, not how fast it travels. This independence of speed from amplitude and frequency (in non-dispersive media) is what makes wave communication reliable — a complex signal composed of many frequencies travels intact without the components arriving at different times.

Question 5

A cork bobs up and down as ocean waves pass beneath it but does not travel toward shore. What does this tell us about what waves actually transport?

Model answer: The cork's behavior shows that waves transport energy, not matter. The water molecules beneath the cork oscillate around their equilibrium positions as each wave passes, temporarily displacing the cork upward and downward, but they do not travel in the direction of wave propagation. Since the cork (which rides on the water surface) does not move toward shore, the water molecules themselves are not flowing shoreward. What moves is the pattern of displacement — the disturbance — which carries energy from the wave source across the ocean without the water itself making that journey.

This is the key conceptual distinction between wave motion and bulk flow. Ocean currents involve water actually flowing from one place to another; waves do not. A student who pictures waves as 'moving water' will misunderstand most wave phenomena — interference, reflection, the independence of wave speed from amplitude. The cork thought experiment isolates this principle cleanly.