Properties of Wave Pulses and Waves

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AP Physics 2 › Properties of Wave Pulses and Waves

Questions 1 - 10
1

A transverse wave travels along a taut rope with amplitude $A$ and speed $v$. The driver’s frequency is doubled while the rope tension and linear density stay constant. Which statement best describes the new wavelength?

It stays the same because wavelength depends only on amplitude.

It increases because higher frequency means higher energy, so crests spread out.

It is halved because the wave speed stays constant in the same medium.

It doubles because the wave speed doubles when frequency doubles.

Explanation

This question tests understanding of properties of wave pulses and waves. For waves on a string, the wave speed v is determined by the medium properties (tension and linear density), which remain constant here. The fundamental wave relationship is v = fλ, where f is frequency and λ is wavelength. When frequency doubles and wave speed stays constant, the wavelength must be halved to maintain this relationship. Choice A incorrectly assumes wave speed depends on frequency, reflecting the misconception that the source controls wave speed rather than the medium. When analyzing wave behavior, always remember that wave speed is set by the medium, not the source.

2

A sinusoidal wave on a string has speed $12\ \text{m/s}$ and wavelength $3.0\ \text{m}$. The amplitude is doubled while the string and tension remain unchanged. Which quantity changes for the wave?

Only the wave speed increases.

Only the energy transported increases.

Only the frequency decreases.

Only the wavelength increases.

Explanation

This question tests understanding of properties of wave pulses and waves. Wave speed on a string depends only on the medium properties (tension and linear mass density), not on amplitude, frequency, or wavelength. Since the string and tension remain unchanged, the wave speed stays at 12 m/s regardless of amplitude changes. The frequency f = v/λ = 12/3 = 4 Hz also remains constant because both speed and wavelength are unchanged. However, wave energy is proportional to the square of amplitude (E ∝ A²), so doubling the amplitude quadruples the energy transported by the wave. Choice B incorrectly assumes amplitude affects wave speed, conflating wave properties with energy transport. Remember: amplitude affects energy but not speed, frequency, or wavelength.

3

A continuous wave travels along a string with wavelength $\lambda=0.60,\text{m}$ and speed $v=9.0,\text{m/s}$. The string tension is adjusted so the wave speed becomes $6.0,\text{m/s}$ while the driver frequency stays constant. Which statement best describes the new wavelength?

It stays $0.60,\text{m}$ because wavelength depends only on amplitude.

It becomes $0.40,\text{m}$ because wavelength is proportional to wave speed at fixed frequency.

It becomes $0.90,\text{m}$ because wavelength is inversely proportional to wave speed.

It becomes $0.30,\text{m}$ because the frequency halves when the speed decreases.

Explanation

This question tests understanding of properties of wave pulses and waves. The wave relationship v = fλ governs how wavelength changes when wave speed changes. Initially, the frequency is f = v/λ = 9.0/0.60 = 15 Hz. When tension is adjusted to reduce wave speed to 6.0 m/s while the driver frequency remains constant at 15 Hz, the new wavelength becomes λ = v/f = 6.0/15 = 0.40 m. Choice C incorrectly assumes wavelength depends only on amplitude, confusing wave propagation properties with wave energy. Always use v = fλ to relate changes in wave speed, frequency, and wavelength.

4

A sinusoidal wave on a string has frequency $5.0,\text{Hz}$ and wavelength $0.40,\text{m}$. The string is replaced with one where waves travel twice as fast. Which quantity changes when the same driver continues oscillating?

The wavelength doubles while the frequency stays the same.

Both frequency and wavelength stay the same because the driver is unchanged.

The amplitude halves because faster waves carry less energy.

The frequency doubles while the wavelength stays the same.

Explanation

This question tests understanding of properties of wave pulses and waves. The wave relationship v = fλ connects wave speed, frequency, and wavelength. When the string is replaced with one where waves travel twice as fast, the wave speed doubles from 2.0 m/s to 4.0 m/s. Since the same driver continues oscillating, the frequency remains 5.0 Hz (the source determines frequency). To maintain v = fλ with doubled speed and constant frequency, the wavelength must double from 0.40 m to 0.80 m. Choice A incorrectly assumes frequency changes, reflecting the misconception that frequency depends on the medium rather than the source. Always remember that frequency is set by the source, while wavelength adjusts based on the medium's wave speed.

5

A sinusoidal water wave moves from deep water into a shallow region where its speed decreases. Which statement best describes what happens to its frequency?

It increases because the wavelength decreases in shallow water.

It remains the same because the source sets the frequency.

It decreases because the wave slows down in shallow water.

It becomes zero because the wave transfers energy without oscillation.

Explanation

This question tests understanding of properties of wave pulses and waves. When a wave travels from one medium to another, its frequency remains constant because frequency is determined by the source of the wave, not the medium through which it travels. The wave relationship v = fλ shows that if speed v decreases (as stated for shallow water) and frequency f stays constant, then wavelength λ must decrease proportionally. The frequency represents how many wave crests the source produces per second, which doesn't change just because the wave enters a different medium. Choice A incorrectly assumes frequency changes with wave speed, confusing the effect of the medium on different wave properties. Remember: frequency is set by the source and remains constant across boundaries, while speed and wavelength change together.

6

A pulse travels along a uniform rope. The tension is increased while the rope’s linear mass density stays constant. Which statement best describes the pulse speed?

It stays the same because pulse speed depends only on amplitude.

It increases because wave speed depends on tension in the rope.

It decreases because higher tension reduces oscillations.

It becomes zero because energy transfer requires interference.

Explanation

This question tests understanding of properties of wave pulses and waves. The speed of a wave pulse on a rope is determined by the rope's physical properties according to v = √(T/μ), where T is tension and μ is linear mass density. When tension increases while mass density stays constant, the wave speed increases because speed is proportional to the square root of tension. This relationship comes from balancing the restoring force (tension) against the inertia (mass per length) of the rope. The pulse amplitude doesn't affect its speed; this is a property of the medium alone. Choice C incorrectly assumes pulse speed depends on amplitude rather than the medium's properties, confusing energy content with propagation characteristics. Remember: wave and pulse speeds depend on medium properties (tension, density), not on wave characteristics (amplitude, frequency).

7

A pulse on a rope carries energy $E$. The amplitude is tripled while the rope tension and linear density stay the same. Which statement best describes the energy carried?

It becomes $9E$ because energy is proportional to amplitude squared.

It becomes $3E$ because energy is proportional to amplitude.

It stays $E$ because energy depends only on wave speed.

It becomes $E/3$ because larger amplitude reduces frequency.

Explanation

This question tests understanding of properties of wave pulses and waves. The energy carried by a wave pulse is proportional to the square of its amplitude: E ∝ A². This relationship holds for all mechanical waves because energy is stored in the displacement from equilibrium, and both kinetic and potential energy scale with displacement squared. When amplitude is tripled (A → 3A), the energy becomes E → 9E because (3A)² = 9A². The rope's tension and density determine the wave speed but don't change the amplitude-energy relationship. Choice A incorrectly assumes a linear relationship between amplitude and energy, missing the quadratic dependence that arises from the physics of oscillatory motion. Remember: wave energy is proportional to amplitude squared (E ∝ A²), making amplitude changes have dramatic effects on energy transport.

8

Two waves travel in the same rope under the same tension: Wave 1 has frequency $f$ and Wave 2 has frequency $2f$. Which statement best compares their speeds?

Wave 1 is faster because lower frequency waves lose less energy.

Their speeds differ unless their amplitudes are the same.

Wave 2 is faster because higher frequency waves travel faster.

They have equal speed because the medium determines wave speed.

Explanation

This question tests understanding of properties of wave pulses and waves. Wave speed in a rope depends only on the rope's physical properties: tension T and linear mass density μ, according to v = √(T/μ). Since both waves travel in the same rope under the same tension, they experience identical medium conditions and therefore have the same speed. The frequency of a wave is set by the source and doesn't affect how fast the wave propagates through the medium. Wave 2 has twice the frequency of Wave 1, which means it has half the wavelength (since v = fλ and v is constant), but both waves travel at the same speed. Choice A incorrectly assumes higher frequency waves travel faster, confusing the rate of oscillation with the rate of propagation. Remember: all waves in the same medium under the same conditions travel at the same speed, regardless of their frequency or amplitude.

9

A wave has speed $v$ in a medium. If its frequency is tripled while the medium is unchanged, which statement best describes the wavelength?

It triples because wavelength is proportional to frequency.

It becomes one-third as large because $\lambda=v/f$.

It stays the same because speed changes to match frequency.

It becomes zero because increasing frequency stops oscillations.

Explanation

This question tests understanding of properties of wave pulses and waves. The wave equation v = fλ can be rearranged to λ = v/f, showing that wavelength is inversely proportional to frequency when wave speed is constant. Since the medium is unchanged, the wave speed v remains constant. When frequency is tripled (f → 3f), the wavelength becomes λ = v/(3f) = (1/3)(v/f), or one-third of its original value. Choice A incorrectly assumes direct proportionality between wavelength and frequency, which would violate the wave equation. When frequency changes but the medium stays the same, wavelength must change inversely to maintain constant wave speed.

10

A periodic wave on a rope has frequency $f=5.0\ \text{Hz}$ and wavelength $\lambda=0.80\ \text{m}$. The rope tension is increased so the wave speed doubles while the source frequency stays constant. Which quantity changes?

The frequency doubles.

The wavelength doubles.

The period doubles.

The amplitude doubles.

Explanation

This question tests understanding of properties of wave pulses and waves. The wave equation v = fλ relates wave speed, frequency, and wavelength. When tension increases and wave speed doubles from 4.0 m/s to 8.0 m/s while the source maintains constant frequency at 5.0 Hz, the wavelength must change to satisfy the wave equation: λ_new = v_new/f = 8.0/5.0 = 1.6 m, which is double the original 0.80 m. Choice B incorrectly suggests frequency doubles, reflecting the misconception that changing the medium properties affects the source frequency. Remember that the source controls frequency, while wavelength adjusts to maintain v = fλ when wave speed changes.

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