Practice question

**Relative motion** changes effective wavelength encountered. For moving source approaching stationary observer, wavelength ahead λ' = (v - v_s)/f, so f' = v/λ' = f·v/(v - v_s) > f, basis for calculating apparent pitch shift in sound. Amplitude: A = 2a cos (Φ/2) , a = 4 m , Φ = (π/6) . A = 2 × 4 cos (π/12) ≈ 8 × 0.966 = 7.73 m . Using v = fλ and standing-wave condition fₙ = n v/(2L) or v/(4L) as applicable, calculation yields 7.7 m, illustrating frequency-length-speed interdependence and quantization by boundaries.