Kmity, vlny, akustika
What you must know
Simple harmonic motion (SHM) has displacement x = A·sin(ωt), with ω = 2π/T = 2πf. For a spring-mass system ω = √(k/m); for a pendulum T = 2π√(l/g). Waves carry energy without transporting matter; wave speed v = λf. Know the difference between transverse and longitudinal waves, superposition, resonance, and the basics of sound: pitch = frequency, loudness = amplitude, and the speed of sound in air (~340 m/s).
Traps
- Confusing period T with frequency f (f = 1/T).
- Thinking the pendulum period depends on mass - it does not.
- Mixing up wavelength and amplitude.
- Forgetting that sound needs a medium; it cannot travel in a vacuum.
- Using degrees where radians are required in ω = 2π/T.
Worked example
A wave has frequency 50 Hz and wavelength 4 m. Its speed is v = λf = 4 × 50 = 200 m/s. If this were a sound wave in air (v ≈ 340 m/s), the same frequency would give λ = 340/50 = 6.8 m. This contrast shows why v = λf is a favourite exam calculation.
Rule of thumb: in SHM, acceleration is always directed toward equilibrium and is maximum at the extremes, where velocity is zero. Energy sloshes between kinetic and potential but the total stays constant in an ideal oscillator.