CBSE Class 9 Physics Waves and Sound MCQs Set 19

Multiple Choice Questions (MCQs) for Class 9 Physics: Waves and Sound

Access targeted multiple-choice questions for Waves and Sound designed to align with the latest CBSE academic syllabus for Class 9 Physics. These objective practice sets help students evaluate their conceptual understanding and improve exam readiness.

Practice Waves and Sound MCQs for Class 9 Physics

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Multiple Choice Questions

Question 1: The distance between a crest and the next trough in a periodic wave is
(a) λ
(b) λ/2
(c) λ/4
(d) 2λ
Show Answer & Explanation

Answer: (b) λ/2

Explanation:
1. Two successive crests are λ apart.
2. A trough lies midway between two crests, so crest to trough is λ/2.

Teacher's Note:
1. Crest to crest is λ; crest to trough is λ/2.

Question 2: In which of the following media will sound travel the fastest?
(a) solid
(b) both solid and liquid
(c) liquid
(d) gas
Show Answer & Explanation

Answer: (a) solid

Explanation:
1. Sound travels fastest in solids because of their greater elasticity, then liquids, then gases.

Teacher's Note:
1. Typical speeds: iron 5130 m/s, water 1500 m/s, air 340 m/s.

Question 3: The wavelength of the B.B.C. broadcast is 15 m. Find the frequency of the waves broadcast by it if the velocity of radio waves is 3 × 108 m/s.
(a) 20 MHz
(b) 30 MHz
(c) 200 MHz
(d) 50 MHz
Show Answer & Explanation

Answer: (a) 20 MHz

Explanation:
1. f = c/λ = 3 × 108 / 15 = 2 × 107 Hz.
2. This is 20 MHz.

Teacher's Note:
1. 1 MHz = 106 Hz.

Question 4: The graph between L and T² for a simple pendulum is
(a) T²L
(b) T²L
(c) T²L
(d) T²L
Show Answer & Explanation

Answer: (a) T²L

Explanation:
1. T² = (4π²/g)L, so T² is directly proportional to L.
2. The graph is a straight line through the origin with a positive slope.

Teacher's Note:
1. A straight line through the origin shows direct proportion.

Question 5: The kinetic energy of the bob of a simple pendulum is maximum at the
(a) mean position
(b) extreme left position
(c) extreme right position
(d) half of the extreme position
Show Answer & Explanation

Answer: (a) mean position

Explanation:
1. At the mean position the bob has the greatest speed.
2. At the extremes the bob is momentarily at rest, so its kinetic energy is zero and its potential energy is greatest.

Teacher's Note:
1. Energy changes between kinetic and potential during the swing.

Question 6: The equation y = a cos²(2πnt − 2πx/λ) represents a wave with
(a) amplitude a, frequency n and wavelength λ
(b) amplitude a, frequency 2n and wavelength 2λ
(c) amplitude a/2, frequency n and wavelength λ
(d) amplitude a/2, frequency 2n and wavelength λ/2
Show Answer & Explanation

Answer: (d) amplitude a/2, frequency 2n and wavelength λ/2

Explanation:
1. cos²θ = (1 + cos 2θ)/2, so y = a/2 + (a/2) cos(4πnt − 4πx/λ).
2. The wave part has amplitude a/2.
3. Comparing with cos(2πft − 2πx/λ'), f = 2n and λ' = λ/2.

Teacher's Note:
1. Use the double-angle formula.

Question 7: The force which tries to bring the body back to its mean position is called
(a) deforming force
(b) restoring force
(c) gravitational force
(d) buoyant force
Show Answer & Explanation

Answer: (b) restoring force

Explanation:
1. The force that acts towards the mean position and opposes the displacement is the restoring force.

Teacher's Note:
1. Restoring force is the cause of oscillations.

Question 8: The oscillations of a pendulum slow down due to the force exerted by
(a) air and friction at the support
(b) air only
(c) friction at the support only
(d) gravity
Show Answer & Explanation

Answer: (a) air and friction at the support

Explanation:
1. Air resistance and friction at the point of suspension remove energy from the pendulum.
2. This reduces the amplitude, so the oscillations die down.
3. Gravity is the restoring force and does not slow the oscillations.

Teacher's Note:
1. These dissipating forces cause damping.

Question 9: Sound travels at a speed of 332 m/s in air. This means that
(a) the source of sound moves 332 m in one second
(b) the listener moves 332 m in one second
(c) air moves 332 m in one second
(d) the disturbance in air moves 332 m in one second
Show Answer & Explanation

Answer: (d) the disturbance in air moves 332 m in one second

Explanation:
1. The speed of sound is the speed at which the disturbance travels through the medium.
2. The air itself, the source and the listener do not move that distance.

Teacher's Note:
1. Only the disturbance travels.

Question 10: A boat at anchor is rocked by waves whose consecutive crests are 0.1 km apart. The wave velocity of the moving crests is 72 km/h. What is the frequency of rocking of the boat?
(a) 0.4 Hz
(b) 0.2 Hz
(c) 0.6 Hz
(d) 0.5 Hz
Show Answer & Explanation

Answer: (b) 0.2 Hz

Explanation:
1. λ = 0.1 km = 100 m and v = 72 km/h = 20 m/s.
2. f = v/λ = 20/100 = 0.2 Hz.

Teacher's Note:
1. Convert km/h to m/s by dividing by 3.6.

Question 11: The distance between two successive crests is 1 m and the velocity of the waves is 320 m/s. What is the frequency?
(a) 340 Hz
(b) 400 Hz
(c) 320 Hz
(d) 420 Hz
Show Answer & Explanation

Answer: (c) 320 Hz

Explanation:
1. λ = 1 m.
2. f = v/λ = 320/1 = 320 Hz.

Teacher's Note:
1. Use v = fλ.

Question 12: A wave travelling in a material medium is described by the equation y = A sin(kx − ωt). The maximum particle velocity is
(a) Aω
(b) ω/k
(c) dω/dk
(d) x/t
Show Answer & Explanation

Answer: (a) Aω

Explanation:
1. Particle velocity = dy/dt = −Aω cos(kx − ωt).
2. Its maximum magnitude is Aω.

Teacher's Note:
1. ω/k is the wave speed, not the particle speed.

Question 13: The wavelength of a sound wave in air is 100 cm. Taking the speed of sound in air as 330 m/s, its frequency is
(a) 33 Hz
(b) 330 Hz
(c) 100 Hz
(d) 3.3 Hz
Show Answer & Explanation

Answer: (b) 330 Hz

Explanation:
1. λ = 100 cm = 1 m.
2. f = v/λ = 330/1 = 330 Hz.

Teacher's Note:
1. Convert cm to m before dividing.

Question 14: If a pendulum is allowed to oscillate in a jar containing water, its time period will
(a) increase
(b) decrease
(c) remain the same
(d) can't say
Show Answer & Explanation

Answer: (a) increase

Explanation:
1. Buoyancy of the water reduces the effective weight of the bob, which is like a smaller effective g.
2. T = 2π√(L/g), so a smaller effective g gives a larger time period.
3. The drag also slows the motion.

Teacher's Note:
1. Smaller effective g means a longer period.

Question 15: A source is producing 15 waves in 3 seconds. The distance between a crest and the adjacent trough is 10 cm. The wavelength is
(a) 20 cm
(b) 30 cm
(c) 40 cm
(d) 50 cm
Show Answer & Explanation

Answer: (a) 20 cm

Explanation:
1. The crest-to-trough distance is λ/2 = 10 cm.
2. So λ = 20 cm. (The frequency, 5 Hz, is not needed for the wavelength.)

Teacher's Note:
1. Crest to trough is half a wavelength.

Question 16: Mechanical waves having a frequency above the audible range are called
(a) sonic
(b) ultrasonic
(c) infrasonic
(d) supersonic
Show Answer & Explanation

Answer: (b) ultrasonic

Explanation:
1. The audible range is 20 Hz to 20,000 Hz.
2. Waves above 20,000 Hz are ultrasonic.

Teacher's Note:
1. Infrasonic waves are below 20 Hz.

Question 17: Imagine a cannon being fired on the surface of the moon. The sound will
(a) be heard at the surface of the earth during all seasons
(b) not be heard at the surface of the earth
(c) be heard at the surface of the earth during the rainy season
(d) not be heard on the earth or on the moon
Show Answer & Explanation

Answer: (b) not be heard at the surface of the earth

Explanation:
1. Sound is a mechanical wave and needs a material medium.
2. There is no air between the moon and the earth, and the moon has almost no atmosphere.
3. So the sound does not reach the earth.

Teacher's Note:
1. Sound cannot travel through vacuum.

Question 18: The velocity of sound in any gas depends upon
(a) wavelength of sound only
(b) density and elasticity of gas
(c) intensity of sound waves only
(d) amplitude and frequency of sound
Show Answer & Explanation

Answer: (b) density and elasticity of gas

Explanation:
1. The speed of a wave in any medium depends on the elasticity and the density of the medium.
2. It does not depend on the wavelength, the intensity or the amplitude.

Teacher's Note:
1. Speed depends on the medium, not the source.

Question 19: If the mass of a pendulum is doubled, then its time period
(a) becomes double
(b) becomes half
(c) is cut by a factor of √2
(d) remains the same
Show Answer & Explanation

Answer: (d) remains the same

Explanation:
1. T = 2π√(L/g) does not contain the mass.
2. So doubling the mass has no effect on the time period.

Teacher's Note:
1. Period depends on length and g only.

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FAQs

Where can I access latest CBSE Class 9 Physics Waves and Sound MCQs Set 19?

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