Progressive waves and their equation, particle velocity against wave velocity, the four speed formulas, and the Newton–Laplace correction with every factor that does and does not change the speed of sound in air.
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 also the chapter the whole unit is built on: Stationary waves and Acoustic phenomena both assume you can read a progressive-wave equation and quote a speed formula without hesitating.
The MEC scope lines are: Progressive waves; Velocity of sound in solids, liquids and gases and the factors affecting it. The two headings below are those points, in MEC's order.
Three ways it is asked: recall (which formula carries the bulk modulus, what γ is for air, the speed of sound in water); understanding (why raising the pressure of air at constant temperature leaves the speed alone, why Newton's answer was 16 % too small, why a wave carries energy but not matter); application (pull f, λ, v and the maximum particle velocity out of a given y = A sin(ωt − kx), scale a speed from 0 °C to 27 °C, find the tension a string needs for a stated wave speed).
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