How two opposed waves build nodes and antinodes, the four laws of a vibrating string, the closed-against-open-pipe table with end correction, the resonance tube and beats.
Unit 3 carries 8 of the 50 physics marks; with six chapters sharing them, this one averages a little over one (MEC publishes weights by unit, not by chapter). It is the most formula-dense chapter of the sound half, and almost all of it reduces to one table and four laws.
The MEC scope line is: Velocity, harmonics and overtones in pipes and strings. That single point is the heading below; its sub-headings follow the order a paper works through it — how a stationary wave forms, strings, pipes, the resonance tube, beats.
Three ways it is asked: recall (which harmonics a closed pipe allows, what the end correction is, the distance between two consecutive nodes); understanding (why a closed pipe sounds an octave below an open pipe of the same length, why the second law of a vibrating string carries a square root, why a stationary wave transports no energy); application (find v from a resonance tube's two lengths, find the tension that doubles a wire's fundamental, find a pipe length from a stated overtone, decide a fork's frequency from a beat count).
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