CBSE Class 10 Physics Waves and Sound MCQs Set 28

Answer: (a) they produce sounds of the same frequency

Explanation:
1. Resonance occurs when the frequency of an applied periodic force equals the natural frequency of the system.
2. Both sources then vibrate with the same frequency.

Teacher's Note:
1. The amplitude of vibration becomes very large at resonance.

Answer: (a) damping

Explanation:
1. Energy loss to the surroundings reduces the amplitude.
2. This process is called damping.

Teacher's Note:
1. Damped oscillations die out with time.

Answer: (d) compression and rarefaction pulses alternating with each other

Explanation:
1. A sound wave is a train of compressions and rarefactions arranged alternately.

Teacher's Note:
1. A compression is followed by a rarefaction and then again a compression.

Answer: (c) both have the same speed

Explanation:
1. In the same medium all sound waves travel at the same speed whatever their frequency.

Teacher's Note:
1. Speed depends on the medium only.

Answer: (b) reverberation

Explanation:
1. Repeated reflections of sound from clouds and the ground cause a prolonged rolling sound.
2. This is called reverberation.

Teacher's Note:
1. Reverberation is the persistence of sound due to multiple reflections.

Answer: (c) at right angles to the direction of wave propagation

Explanation:
1. In a transverse wave the particle motion is perpendicular to the direction in which the wave moves.

Teacher's Note:
1. Water ripples and waves on a rope are transverse.

Answer: (b) rhinoceroses

Explanation:
1. Infrasound is below 20 Hz, which humans cannot hear.
2. Rhinoceroses can produce and hear very low frequencies.

Teacher's Note:
1. Elephants and rhinoceroses use infrasound to communicate.

Answer: (d) v\( _1 \) = v\( _2 \) = v\( _3 \)

Explanation:
1. The speed of sound in a given medium is the same for every frequency.

Teacher's Note:
1. Only the medium decides the speed.

Answer: (d) all of the above

Explanation:
1. In a compression the particles are crowded together.
2. So the volume of a given mass falls, and density and pressure rise.

Teacher's Note:
1. A rarefaction shows the opposite changes.

Answer: (a) a material medium as well as vacuum

Explanation:
1. Electromagnetic waves are non-mechanical.
2. They travel through vacuum and also through transparent material media like air and glass.

Teacher's Note:
1. Mechanical waves such as sound need a medium.

Answer: (b) 1.6 \( \times \) 10\( ^{-3} \) m, 6.8 \( \times \) 10\( ^{-3} \) m

Explanation:
1. The reflected sound stays in air: \( \lambda \) = 352/220000 = 1.6 \( \times \) 10\( ^{-3} \) m.
2. The transmitted sound is in water: \( \lambda \) = 1496/220000 = 6.8 \( \times \) 10\( ^{-3} \) m.

Teacher's Note:
1. The frequency stays the same when sound passes between media; the speed and wavelength change.

Answer: (b) a cone

Explanation:
1. A source moving faster than sound produces a conical shock wave (Mach cone).

Teacher's Note:
1. The cone becomes narrower as speed increases.

Answer: (a) amplitude

Explanation:
1. Amplitude is the maximum displacement from the mean position.
2. The depth of a trough and the height of a crest are both equal to the amplitude.

Teacher's Note:
1. Amplitude decides the loudness of sound.

Answer: (b) reverberation of sound

Explanation:
1. Repeated reflections make sound persist and become confused.
2. Absorbent materials reduce this reverberation.

Teacher's Note:
1. Curved ceilings and soundboards help the sound to reach all listeners.

Answer: (b) III, I, II

Explanation:
1. Sound is slowest in gases (nitrogen, about 350 m/s).
2. It is faster in liquids (water, about 1500 m/s).
3. It is fastest in solids (steel, about 5000 m/s).

Teacher's Note:
1. Speed is generally solids > liquids > gases.

Answer: (a) rarefaction

Explanation:
1. Particles that are spread apart make a region of low density, which is a rarefaction.

Teacher's Note:
1. Crest and trough are terms for transverse waves only.

Answer: (b) 14.47 s

Explanation:
1. The signal goes down and comes back, covering 2 \( \times \) 11000 = 22000 m.
2. t = 22000/1520 = 14.47 s.

Teacher's Note:
1. Echo time = 2d/v.

Answer: (a) crests and troughs in the medium

Explanation:
1. A crest is the highest point and a trough is the lowest point of the wave.
2. Compressions and rarefactions occur only in longitudinal waves.

Teacher's Note:
1. Transverse waves cannot travel in gases.

Answer: (b) transverse wave

Explanation:
1. The particles vibrate at right angles to the direction in which the disturbance travels along the wire.
2. This is the definition of a transverse wave.

Teacher's Note:
1. A wave on a stretched string is transverse.

Answer: (a) slow down

Explanation:
1. T = 2\( \pi \sqrt{l/g} \).
2. g on the Moon is about one sixth of that on the Earth, so T becomes larger.
3. The pendulum swings more slowly.

Teacher's Note:
1. A lower g makes a longer time period.

Answer: (c) atmospheric pressure

Explanation:
1. Speed depends on temperature, humidity and the medium.
2. A change of pressure alone, at fixed temperature, changes density in the same proportion, so the speed stays the same.

Teacher's Note:
1. Speed is affected by pressure only through temperature and composition.

Answer: (a) 1.5 m

Explanation:
1. The distance between two successive compressions is one wavelength, 3.0 m.
2. A rarefaction centre lies midway between two compressions.
3. The required distance is \( \lambda \)/2 = 1.5 m.

Teacher's Note:
1. Compression to compression is \( \lambda \); compression to rarefaction is \( \lambda \)/2.

Answer: (d) the medium must have a large density

Explanation:
1. A source, a medium and a receiver are required.
2. Large density of the medium is not necessary (sound travels in air, which has a low density).

Teacher's Note:
1. The ear and brain together act as receiver.