Grade 12 · CAPS

Grade 12 Physics: Waves and sound

Physical Sciences practice questions and a worked example

Grade 12 Physics: Waves and Sound is part of the CAPS Physical Sciences curriculum, covering the principles of wave phenomena and sound. Learners will explore concepts such as the Doppler effect, redshift, and the calculation of frequency and velocity in various scenarios. These skills are essential for understanding how waves behave in different contexts, such as moving vehicles and astronomical observations. Through the practice questions provided, students will engage with real-world applications of wave theory, including analyzing the movement of stars and calculating the speed of sound. The questions below are drawn from ReClass's own question bank, offering a comprehensive practice experience.

Worked example

6.2 A stationary listener, standing on the roadside, records the wavelength of the sound emitted by the siren of a police car travelling at a constant velocity. In the wavelength-time graph (NOT drawn to scale), λL, the wavelength recorded by the listener, is 0,40 m and λS, the wavelength emitted by the siren, is 0,38 m. Take the speed of sound in air to be 343 m·s-1. Is the car moving TOWARDS or AWAY FROM the listener? Give a reason for the answer.

Answer & explanation

AWAY FROM the listener. The wavelength detected by the listener (0,40 m) is longer than the wavelength emitted by the siren (0,38 m), i.e. λL > λS (frequency detected is lower).

When a sound source moves away from a listener, the sound waves are stretched, resulting in a longer wavelength and a lower frequency. This is known as the Doppler effect. In this case, the wavelength recorded by the listener (

λ_{L} = 0.40 m

) is longer than the wavelength emitted by the siren (

λ_{S} = 0.38 m

). This indicates that the frequency detected by the listener is lower than the frequency emitted, which happens when the source is moving away. Therefore, the car is moving away from the listener.

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Physics: Waves and sound

Question 1 of 3

1.6 The absorption spectrum of an element surrounding a moving star is observed on Earth and found to be red shifted. Which ONE of the following combinations is CORRECT for the movement of the star and the frequency of the observed light on Earth?

AAway from Earth; decreasedBTowards Earth; decreasedCAway from Earth; increasedDTowards Earth; increased

Learn from each answer, right or wrong.

Try again before revealing the solution.

Common mistakes

Confusing redshift with blueshift when analyzing star movement.
Incorrectly identifying the direction of a moving object based on wavelength changes.
Misapplying the Doppler effect formula when calculating frequency or velocity.
Assuming the speed of sound is constant in all conditions without considering environmental factors.

Aligned to CAPS Grade 12 Physical Sciences — Physics: Waves and sound. Curriculum source: CAPS for the FET Phase, Department of Basic Education.