Practice question

**Doppler effect** describes apparent frequency shift due to relative motion between source and observer, f' = f·v/(v ∓ v_s) for source motion, f' = f·(v ± v_o)/v for observer motion, upper signs for approach increasing observed frequency. Motion towards observer compresses wavelength raising f'. Let v₂ be the original frequency. |320 - v₂| = 4 ⇒ v₂ = 316 Hz or 324 Hz . Increasing tension increases frequency. If v₂ = 316 , new v₂’ > 316 , beat = 320 - v₂’ < 4 , becomes 2 Hz ( v₂’ = 318 ), consistent. If v₂ = 324 , beat increases, contradicts. So, v₂