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 = 3 m , Φ = (π/4) . A = 2 × 3 cos (π/8) ≈ 6 × 0.9239 ≈ 5.54 m . Using v = fλ and standing-wave condition fₙ = n v/(2L) or v/(4L) as applicable, calculation yields 5.5 m, illustrating frequency-length-speed interdependence and quantization by boundaries.