Practice Waves and Sound: Physics Unit 6.
This unit covers wave properties, sound waves, Doppler effect and resonance — essential concepts for Physics. Use our interactive study games to test your understanding, or review questions in traditional format below.
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This unit covers wave properties, sound waves, Doppler effect and resonance — essential concepts for Physics. Use our interactive study games to test your understanding, or review questions in traditional format below.
Key Concepts Breakdown
1 Wave Properties
Students must know the relationship between wave speed, frequency, and wavelength (v = fλ). They must be able to identify and distinguish between transverse and longitudinal waves. Understanding amplitude, period, and how they relate to energy and time is essential for exam questions.
Key Points
- Wave speed equation: v = fλ (speed = frequency × wavelength)
- Transverse waves: particles vibrate perpendicular to wave direction (e.g., light, water waves)
- Longitudinal waves: particles vibrate parallel to wave direction (e.g., sound)
- Amplitude determines energy; frequency determines pitch (for sound) or color (for light)
A wave has a frequency of 200 Hz and a wavelength of 1.5 m. What is its wave speed?
Use v = fλ: v = 200 Hz × 1.5 m = 300 m/s. Frequency is given in hertz (cycles per second) and wavelength in meters, so the product gives meters per second. This is a direct plug-and-chug application of the wave equation.
2 Sound Waves
Sound is a mechanical, longitudinal wave that requires a medium to travel. Students must know that sound travels faster in solids than liquids than gases, and that speed increases with temperature in air. Loudness corresponds to amplitude and pitch corresponds to frequency.
Key Points
- Sound cannot travel through a vacuum — it requires a medium (solid, liquid, or gas)
- Speed of sound in air at 20°C ≈ 343 m/s; faster in water (~1480 m/s) and steel (~5100 m/s)
- Higher frequency = higher pitch; greater amplitude = louder sound
- Compressions (high pressure) and rarefactions (low pressure) make up a sound wave
A student claps near a wall 85 m away and hears an echo 0.5 seconds later. What is the calculated speed of sound?
Sound travels to the wall and back, so total distance = 2 × 85 m = 170 m. Using v = d/t: v = 170 m ÷ 0.5 s = 340 m/s. The key detail students miss is doubling the distance since the echo involves a round trip.
3 Doppler Effect
The Doppler effect is the perceived change in frequency (pitch) when a wave source and observer are moving relative to each other. When the source moves toward the observer, the perceived frequency increases; when it moves away, it decreases. Students do not need to use the formula at the standard level but must understand the conceptual cause and direction of the shift.
Key Points
- Source moving toward observer → wavelengths compress → higher perceived frequency (higher pitch)
- Source moving away from observer → wavelengths stretch → lower perceived frequency (lower pitch)
- The actual frequency emitted by the source does NOT change — only the perceived frequency changes
- Real-world applications: ambulance siren, radar speed guns, red-shift in astronomy
A fire truck sounding its siren at a constant frequency drives toward a stationary person, then passes and drives away. How does the pitch the person hears change?
As the truck approaches, sound waves bunch up in front of the truck, so the person hears a higher pitch than the siren's true frequency. After the truck passes and moves away, the waves spread out, and the person hears a lower pitch. The siren's actual frequency never changed — only the observer's perception shifted due to relative motion.
4 Resonance
Resonance occurs when an object is driven at its natural frequency, causing it to vibrate with maximum amplitude. Students must understand standing waves in open and closed pipes and on strings, including where nodes (no movement) and antinodes (maximum movement) form. The fundamental frequency and its harmonics are common exam targets.
Key Points
- Resonance: maximum energy transfer occurs when driving frequency matches the object's natural frequency
- Standing waves form through interference of two waves with the same frequency traveling in opposite directions
- Nodes = points of no displacement; antinodes = points of maximum displacement
- Fundamental (1st harmonic) has the lowest resonant frequency; higher harmonics are integer multiples of it
A string fixed at both ends is 0.6 m long and vibrates in its fundamental mode. Where are the nodes and antinodes, and what is the wavelength of the standing wave?
In the fundamental mode (1st harmonic), there is one antinode in the middle and nodes at each fixed end. The string length equals half a wavelength (L = λ/2), so λ = 2 × 0.6 m = 1.2 m. Students must remember that both ends are nodes because the string cannot move where it is fixed.
Questions, answered.
What is Waves and Sound?
Waves and Sound is Unit 6 of Physics, covering wave properties, sound waves, Doppler effect and resonance.
How to study for Physics Unit 6?
Start with the Quick Summary above, review the Key Concepts, then test yourself with our interactive study games. Aim for 80%+ accuracy before moving on.
How many questions are in this unit?
This unit has 27+ review questions across 5 different game modes.