PHY 1020 – Chapter 7: Waves, UFOs, Earthquakes, and Music

Question-and-answer flashcards covering definitions, formulas, and phenomena related to waves, sound, Doppler effect, SOFAR channel, earthquakes, and tsunamis.

What is a wave?

A disturbance or oscillation that transfers energy through space and matter.

Which two main quantities describe a wave’s size and strength?

Amplitude (displacement/density) and wavelength (λ, size of the wave).

How can you determine the velocity of a wave from observation?

Measure the time it takes one wavelength to pass a point (period) and use v = λ / T.

What is the mathematical definition of a wave’s period (T)?

T = λ / v – the time for one full wavelength to pass a point.

What is the mathematical definition of wave frequency (f)?

f = v / λ – the number of wavelengths that pass a point each second.

How do longitudinal (compression) waves move particles?

Parallel to the direction of wave propagation.

How do transverse waves move particles?

Perpendicular to the direction of wave propagation.

What happens when two waves meet according to the superposition principle?

Their displacements add, forming a new resultant wave.

What is constructive interference?

When two waves combine to increase amplitude.

What is destructive interference?

When two waves combine to decrease (or cancel) amplitude.

State Huygens’ Principle spread equation.

S = (λ R) / D, where S is spread, λ wavelength, R distance from opening, and D opening size.

How is sound pitch related to frequency?

Higher frequency corresponds to a higher perceived pitch.

Why does sound travel faster in stiffer materials?

Greater stiffness allows molecules to transmit compression waves more quickly.

Define ‘beats’ in acoustics.

Periodic variations in loudness due to interference of two close frequencies; beat frequency f_B = |f2 – f1|.

Write the decibel intensity formula.

I(dB) = 10 log10 (I / I0), where I0 = 1.0 × 10⁻¹² W/m² (threshold of hearing).

How many decibels correspond to an intensity of 2.5 × 10⁻⁴ W/m²?

≈ 84 dB.

At approximately what decibel level does the threshold of pain begin?

Around 140 dB.

Explain the Doppler effect in one sentence.

It is the observed change in frequency of a wave when the source and observer move relative to each other.

When a siren approaches you, how does its observed frequency change?

It is higher than the emitted frequency.

What is the SOFAR channel?

A horizontal ocean layer where sound speed is minimal, allowing low-frequency sound to travel thousands of miles by refraction.

Name the three main earthquake wave types.

Primary (P) waves, Secondary (S) waves, and Long (L) waves.

Which earthquake waves arrive first and what type are they?

P waves; they are longitudinal compression waves.

What type of motion characterizes S waves and when do they arrive?

Transverse shearing motion; they arrive second after P waves.

Give the magnitude relation for earthquakes in terms of amplitude and distance.

M = log10 (A / A0(d)), where A is measured amplitude and A0(d) a reference amplitude at distance d.

What distinguishes shallow-water from deep-water waves?

Whether the wavelength (L) is large enough to interact with the sea floor relative to water depth.

State the velocity approximation used for tsunami travel time (given depth ≈ 3000 m).

v ≈ √(g·D) ≈ 171 m/s (≈ 386 mph).

Roughly how much warning does a tsunami 1000 miles away provide?

About 2.6 hours.

What is wave run-up height during a tsunami?

The vertical height water reaches above normal sea level when the wave comes ashore.

Why are Hawaiian islands especially concerned with tsunami warning systems?

They lie in the Pacific closer to many tsunami sources and may receive waves within a few hours.