# Mechanical Waves – Short Questions With Answer | Physics Class 12

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## Short Answer Question | Physics Short Question-Answer from Mechanical Waves Class: 12

1. Why sound made at a distance can be heard distinctly at night than in the day time?
Ans. At night, the atmospheric air becomes more moist than at the day time, which reduces the density of air in the night time. As it is known, the velocity of sound is inversely proportional to the square root of the density (i.e. vs 1/ρ). So, at night velocity of sound is more at night than at day due to reduced density of air. Apart from this, the sound absorbing bodies such as people, vehicles and noise from other sound producing bodies are least at night time as compared to the day time. So sound made at a distance can be heard more distinctly at night than at the day time.

2. Is velocity of sound more in damp air or in dry air? Explain.
Ans. Velocity of sound is more in damp air. We know that the density of damp air is less than air due to the presence of humidity. Also we have the relation v 1/(p)^(1/2) i.e. the velocity of sound in air is inversely proportional to the square root of the density of the medium. So the velocity of sound in damp air is more than that of in dry air.

3. Do sound waves need a medium to travel from one point to other point in space? What properties of the medium are relevant?
Ans. Yes, sound wave needs a medium to travel from one point to other point in space. This is because it is longitudinal wave as well as mechanical wave. The relevant properties of the medium are follows:

• The medium should possess the property of elasticity.
• The medium should possess the property of inertia.
• The medium should have minimum friction.

4. Explain why the velocity of sound in solids is greater than that in gases, though the densities of solids are greater than that of gases.
Ans. The sound is longitudinal wave. In general the velocity of longitudinal wave is given by v= (r / p)1/2 , where γ is the modulus of elasticity and ρ is the density of the medium. For the solids, the ratio (r/p) is much greater than that for gases. Hence, the velocity of sound in solids is greater than in gases, although the densities of solids are greater than that of gases.

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