Chapter – Sound (Waves)
Meaning of symbols and Important Formulae
(• Wave velocity: v• Frequency: f
• Frequency: f = 1 / T
• Time period: T = 1 / f
• Speed of sound in air ≈ 330 m s⁻¹
• Speed of sound in water ≈ 1360 m s⁻¹
Numericals
1. Heart beats 75 times per minute. Find frequency and time period.
Frequency = 75 / 60 = 1.25 Hz Time period = 1 / 1.25 = 0.8 s
2. Time period of pendulum is 2 s. Find frequency.
Frequency = 1 / 2 = 0.5 Hz
3. Separation between two crests is 100 m. Velocity = 20 m s⁻¹. Find frequency.
f = v / λ = 20 / 100 = 0.2 Hz
4. Velocity = 0.3 m s⁻¹, frequency = 20 Hz. Find wavelength.
λ = v / f = 0.3 / 20 = 0.015 m
5. 40 crests produced in 0.4 s. Find frequency.
Frequency = 40 / 0.4 = 100 Hz
6. Gun fired. Observer B is 1650 m away. Speed = 330 m s⁻¹. Find time.
Time = 1650 / 330 = 5 s
7. Lightning and thunder gap = 5 s. Speed = 330 m s⁻¹. Find distance.
Distance = 330 × 5 = 1650 m
8. Gun heard after 2.5 s. Speed = 340 m s⁻¹. Find distance.
Distance = 340 × 2.5 = 850 m
9. Observer hears sounds after 2 s and 3.5 s. Distance = 510 m. Find speed.
Time difference = 1.5 s Speed = 510 / 1.5 = 340 m s⁻¹
10. Time taken by sound to travel 3.3 km in air and iron.
Air: 3300 / 330 = 10 s Iron: 3300 / 5280 = 0.625 s
11. Distance = 1700 m. Speed in air = 340 m s⁻¹, in water = 1360 m s⁻¹.
Air: 1700 / 340 = 5 s Water: 1700 / 1360 = 1.25 s
