MDCAT Physics · Physics · Waves and oscillations. The idea in plain words, the rules to remember, the mistake to avoid and a worked example, written and checked by Mega Lecture.
A stationary wave forms when two waves of the same frequency and amplitude travel in opposite directions and superpose. Points that never move are nodes; points of largest amplitude are antinodes. Neighbouring nodes are half a wavelength apart, and no energy is carried along the wave.
A string fixed at both ends has a node at each end. An open pipe has an antinode at each end. A pipe closed at one end has a node at the closed end and an antinode at the open end, so it gives odd harmonics only.
Sound travels fastest in solids and slowest in gases. Two notes of nearly equal frequency give beats. Relative motion of source and listener changes the frequency heard (Doppler effect).
Giving a closed pipe even harmonics. Its resonant frequencies are f, 3f, 5f, so the third-lowest is 5f, not 3f.
Answer: 680 Hz (higher than 660 Hz, as expected for an approaching source)
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