Speed of Sound | Sound | Assessment | English | Grade 9

Why are supersonic sounds produced in jet aircrafts and bullets?

• Because of its high speed.

• Because the frequency associated with them is quite high.

• Because of the air pressure variations associated with the shock waves.

• None of these.

Answer explanation

Bullets and jet aircrafts travel with speed more than the speed of air, which produces shock waves in the air. These shock waves results in air pressure variation. Thus, creating the supersonic sound.

Suppose 400 waves pass through a point in 10 seconds and the distance between two crests is 10 m. Calculate i) the speed ii) the time period

• i) 20000m/s ii) 0.00005 s

• i) 400m/s ii) 0.025 s

• i) 200m/s ii) 0.025 s

• i) 0.25 m/s ii) 40 s

Answer explanation

Here, Frequency = 400/10 = 40 Hz Speed = λ × Frequency = 10 × 40 = 400m/s Time period = 1/40 = 0.025 s

Which of the following most affects the speed of sound?

• Loudness

• Amplitude

• Temperature

• Pitch

Answer explanation

Temperature affects the speed of sound the most.

Why do you think the arrival of a train can be better predicted by putting an ear to the train track?

• Because wheels of the train touch the track.

• Because sound travels faster in solids than in the air.

• Because density of railway track is more than the density of air.

• Both 2 and 3

Answer explanation

Putting ear to a railway track makes it easier to predict the arrival of train because sound travels faster in solids than gas. Also speed of sound is more when density is more. Here density of railway track is more than the density of the air. Hence option 4 is correct.

Raju dropped a stone from the top of a tower, which is 200 m high. The stone falls into a pond at the base of the tower and a splash sound is heard. When do you think the Raju will hear the sound, if speed of sound is 350 m/s. [Take g= 10 m/s²]

• 0.571 sec

• 6.895 sec

• 40.571 sec

• None of these.

Answer explanation

Distance = 200 m, speed = 350 m/s, u = 0 let time taken by stone to fall be T1 According to second equation of motion, S = ut + 1/2 at² 200 = 0 + 1/2 × 10 × T1² T1² = 400/10 = 40 T1 = √40 = 6.324 sec Time taken by sound to reach from bottom to top be T2 T2 = 200/350 T2 = 0.571 sec Sound is heard after, T1 +T2 = 6.324 + 0.571 = 6.895 seconds

The voice of some singers have the capacity to break glass. Why do you think the glass breaks?

• Because the high pitch of the sound.

• Because the frequency of the sound becomes more than the natural frequency of the glass.

• Because of high note of the sound.

• Because the wavelength of the sound becomes more than the natural wavelength of the glass.

Answer explanation

The glass has a natural frequency with which it vibrates. When the frequency of the sound becomes more the vibrations become enough to break the glass. Hence option 2 is correct