Physique-Chimie — Première

Ondes mécaniques — Première Physique-Chimie

Nature des ondes, célérité, longueur d'onde, période et propagation.

Par ProfBot

Ondes mécaniques — Première Physique-Chimie

🎯 Objectif : Caractériser une onde mécanique et calculer ses grandeurs.

1. Définition

Une onde mécanique = perturbation qui se propage sans transport de matière mais avec transport d'énergie.

2. Représentation d'une onde périodique

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y2="161.0" stroke="#dc2626" stroke-width="2"/> <line x1="228.0" y1="149.0" x2="228.0" y2="161.0" stroke="#dc2626" stroke-width="2"/> <text x="129" y="169" text-anchor="middle" font-size="11" fill="#dc2626" font-weight="bold">T (période)</text> <line x1="79.0" y1="100.0" x2="79.0" y2="65.0" stroke="#16a34a" stroke-width="2"/> <text x="91" y="82" text-anchor="start" font-size="12" fill="#16a34a" font-weight="bold">A</text> <text x="228" y="185" text-anchor="middle" font-size="11" fill="#2563eb" font-weight="normal">λ = v × T = v / f</text>

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Grandeurs caractéristiques :


T = période (s)          f = fréquence (Hz) = 1/T
v = célérité (m/s)       λ = longueur d'onde (m)

Relations fondamentales :


f = 1/T
λ = v × T = v / f
v = λ × f

3. Retard temporel


τ = d / v    (retard pour parcourir la distance d)

Exemple : éclair vu 3 s avant le tonnerre :


d = 340 × 3 = 1 020 m ≈ 1 km

4. Célérités dans différents milieux

MilieuCélérité du son
Air (20°C)340 m/s
Eau1 500 m/s
Acier5 000 m/s

Lumière dans le vide : c = 3,0 × 10⁸ m/s

5. Domaines de fréquence


f < 20 Hz      → Infrasons (inaudibles)
20 Hz–20 kHz   → Sons audibles
f > 20 kHz     → Ultrasons

6. Diffraction

Quand une onde passe par une ouverture de largeur a :


Diffraction notable si λ ≈ a

📝 Exercices

  1. Son de 440 Hz dans l'air (v=340 m/s). Calculer T et λ.
  2. Sonar : écho après 0,6 s. Profondeur ?
  3. Ultrason λ = 2 mm dans l'eau. Calculer f.
  4. Quelle célérité si T = 2 ms et λ = 70 cm ?

✅ À retenir


f = 1/T    λ = v/f    v = λf
Retard : τ = d/v
Son dans l'air : 340 m/s    Lumière : 3×10⁸ m/s
Audible : 20 Hz – 20 kHz

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