Oscillations and Waves — GK Questions with Answers

1. What is the acceleration at the mean position in SHM?
   1. maximum
   2. zero
   3. constant but not zero
   4. negative maximum
   Show answer

   Answer: (B) zero

   At the mean position the displacement is zero and a = −ω²x, so the acceleration is also zero; the velocity is maximum there.

2. What is the phase difference between acceleration and displacement in SHM?
   1. 0
   2. π/2
   3. 2π
   4. π
   Show answer

   Answer: (D) π

   a = −ω²x — the minus sign shows that acceleration is always opposite to displacement, a phase difference of π (180°).

3. If the amplitude of a particle in SHM is doubled, its total energy becomes —
   1. four times
   2. double
   3. half
   4. the same
   Show answer

   Answer: (A) four times

   E = ½kA², so energy is proportional to the square of the amplitude; double the amplitude gives four times the energy.

4. The time period of a simple pendulum does NOT depend on —
   1. length
   2. value of g
   3. mass of the bob
   4. place
   Show answer

   Answer: (C) mass of the bob

   T = 2π√(l/g) has no mass in it — a wooden or iron bob, the time is the same.

5. The length of a seconds pendulum on Earth is about —
   1. 25 cm
   2. 50 cm
   3. 1 m
   4. 2 m
   Show answer

   Answer: (C) 1 m

   For a time period of 2 s, l = gT²/4π² ≈ 0.994 m, that is, about 1 m.

6. If a pendulum clock is taken to the top of a mountain, it will —
   1. become slow
   2. run fast
   3. run the same
   4. stop
   Show answer

   Answer: (A) become slow

   At a height g decreases, so T = 2π√(l/g) increases and the clock starts losing time.

7. Inside a satellite orbiting the Earth, the time period of a simple pendulum will be —
   1. zero
   2. same as on Earth
   3. 2 s
   4. infinite
   Show answer

   Answer: (D) infinite

   In a satellite the effective g is zero; T = 2π√(l/0) is infinite — the pendulum does not oscillate at all.

8. If a mass on a spring is taken to the Moon, its time period —
   1. increases
   2. stays the same
   3. decreases
   4. becomes zero
   Show answer

   Answer: (B) stays the same

   T = 2π√(m/k) has no g in it, so the time period of a spring is the same everywhere.

9. If the length of a pendulum is made four times, its time period becomes —
   1. double
   2. four times
   3. half
   4. the same
   Show answer

   Answer: (A) double

   T ∝ √l, so √4 = 2 times.

10. Why are soldiers asked to break step while crossing a bridge?
    1. to avoid tiredness
    2. to reduce friction
    3. to avoid resonance
    4. to reduce noise
    Show answer

    Answer: (C) to avoid resonance

    If the frequency of the march matches the natural frequency of the bridge, resonance can increase the amplitude a lot and break the bridge.

11. Transverse mechanical waves cannot form in —
    1. a string
    2. a gas
    3. a solid
    4. the surface of a liquid
    Show answer

    Answer: (B) a gas

    A gas has no shear elasticity, so only longitudinal waves travel in it.

12. Sound waves in air are —
    1. transverse
    2. polarised
    3. electromagnetic
    4. longitudinal
    Show answer

    Answer: (D) longitudinal

    In air the particles vibrate to and fro along the direction of the wave and form compressions and rarefactions.

13. Sound goes from air into water. Which quantity does not change?
    1. speed
    2. wavelength
    3. all three change
    4. frequency
    Show answer

    Answer: (D) frequency

    The source decides the frequency; on changing the medium the speed and wavelength change, not the frequency.

14. The wavelength of a 170 Hz sound with speed 340 m/s is —
    1. 0.5 m
    2. 2 m
    3. 20 m
    4. 57800 m
    Show answer

    Answer: (B) 2 m

    λ = v/f = 340/170 = 2 m.

15. Which seismic waves do not pass through the Earth’s outer core?
    1. P-waves
    2. all pass
    3. S-waves
    4. radio waves
    Show answer

    Answer: (C) S-waves

    S-waves are transverse and cannot travel through liquids; their absence showed that the outer core is liquid.

16. If the pressure of air is doubled at constant temperature, the speed of sound —
    1. does not change
    2. halves
    3. becomes √2 times
    4. doubles
    Show answer

    Answer: (A) does not change

    When P rises, density rises in the same ratio; P/ρ stays the same, so the speed does not change.

17. In which medium does sound travel fastest?
    1. air
    2. steel
    3. water
    4. vacuum
    Show answer

    Answer: (B) steel

    Solids have very high elasticity; ≈ 5960 m/s in steel, ≈ 1500 m/s in water, ≈ 346 m/s in air, and sound does not travel in vacuum.

18. Compared with dry air, the speed of sound in moist air is —
    1. less
    2. zero
    3. equal
    4. more
    Show answer

    Answer: (D) more

    Water vapour is lighter than air, so moist air has lower density and sound travels faster.

19. Who corrected Newton’s formula for the speed of sound in gases?
    1. Laplace
    2. Hooke
    3. Doppler
    4. Sabine
    Show answer

    Answer: (A) Laplace

    Laplace said the process in sound is adiabatic and gave v = √(γP/ρ).

20. The pitch of a sound depends on its —
    1. amplitude
    2. wave shape
    3. frequency
    4. speed
    Show answer

    Answer: (C) frequency

    Higher frequency gives higher pitch (shrill sound); amplitude decides loudness and wave shape decides quality.

21. Choose the correct statement about the sounds of a mosquito and a lion.
    1. the lion has higher pitch, the mosquito is louder
    2. both have equal pitch
    3. the mosquito has higher pitch, the lion is louder
    4. both are equally loud
    Show answer

    Answer: (C) the mosquito has higher pitch, the lion is louder

    A mosquito flaps its wings very fast (high frequency), while a lion’s roar has a large amplitude.

22. When the sound level rises by 10 dB, its intensity becomes —
    1. 10 times
    2. double
    3. 100 times
    4. 10 W/m² more
    Show answer

    Answer: (A) 10 times

    β = 10 log(I/I₀) — 10 dB is 1 bel, so the intensity becomes 10 times.

23. Under India’s noise pollution rules, the night limit for residential areas is —
    1. 35 dB
    2. 70 dB
    3. 55 dB
    4. 45 dB
    Show answer

    Answer: (D) 45 dB

    Residential: 55 dB day, 45 dB night; silence zone 50/40, industrial 75/70.

24. The audible range of a normal human is —
    1. 0–20 Hz
    2. 20–20,000 Hz
    3. above 20,000 Hz
    4. 200–2000 Hz
    Show answer

    Answer: (B) 20–20,000 Hz

    Below 20 Hz is infrasonic and above 20 kHz is ultrasonic.

25. How does a bat find its way in the dark?
    1. by echoes of ultrasound
    2. by infrasound
    3. by infrared rays
    4. by magnetic field
    Show answer

    Answer: (A) by echoes of ultrasound

    A bat gives out ultrasonic squeaks and judges distance and direction from their echoes — this is called echolocation.

26. What is used to break kidney stones without surgery?
    1. X-rays
    2. infrasonic waves
    3. ultrasonic waves
    4. gamma rays
    Show answer

    Answer: (C) ultrasonic waves

    In lithotripsy ultrasonic waves break the stone into fine pieces, which pass out with urine.

27. The minimum distance of a reflecting surface for hearing a clear echo is about —
    1. 1.7 m
    2. 17.2 m
    3. 34.4 m
    4. 172 m
    Show answer

    Answer: (B) 17.2 m

    The effect of sound lasts 0.1 s in the ear; d = 344 × 0.1 / 2 ≈ 17.2 m.

28. A SONAR pulse returns after 2 s; the speed of sound in water is 1500 m/s. Depth of the sea?
    1. 750 m
    2. 6000 m
    3. 3000 m
    4. 1500 m
    Show answer

    Answer: (D) 1500 m

    Depth = vt/2 = 1500 × 2 / 2 = 1500 m — the sound goes down and also comes back.

29. Which is the right way to reduce reverberation in an auditorium?
    1. smooth marble walls
    2. a high glass roof
    3. an empty hall
    4. heavy curtains and carpets
    Show answer

    Answer: (D) heavy curtains and carpets

    Curtains, carpets, cushioned seats and fibre board absorb sound and reduce the reverberation time.

30. The whistle of an approaching train is heard by a person on the platform at —
    1. a lower frequency
    2. a higher frequency
    3. the same frequency
    4. not at all
    Show answer

    Answer: (B) a higher frequency

    Doppler effect: when the source approaches, the waves get squeezed and the heard frequency rises.

31. What does the "red shift" in the light of distant galaxies show?
    1. they are coming closer
    2. they are still
    3. they are moving away
    4. they are cooling
    Show answer

    Answer: (C) they are moving away

    A receding source has a lower frequency and its light shifts towards red — this showed that the universe is expanding.

32. Motion with Mach number more than 5 is called —
    1. hypersonic
    2. transonic
    3. supersonic
    4. subsonic
    Show answer

    Answer: (A) hypersonic

    M < 1 subsonic, 1–5 supersonic and more than 5 hypersonic.

33. Where is the smallest bone of the human body?
    1. inner ear
    2. middle ear
    3. nose
    4. finger
    Show answer

    Answer: (B) middle ear

    The stapes (stirrup) of the middle ear is the smallest bone of the body.

34. How many beats per second are heard when tuning forks of 256 Hz and 262 Hz are sounded together?
    1. 3
    2. 518
    3. 256
    4. 6
    Show answer

    Answer: (D) 6

    Beat frequency = |f₁ − f₂| = 262 − 256 = 6.

35. Which harmonics are formed in an organ pipe closed at one end?
    1. only odd
    2. only even
    3. all
    4. only the fundamental
    Show answer

    Answer: (A) only odd

    The condition of a node at the closed end and an antinode at the open end gives only odd harmonics (1 : 3 : 5).

36. Compared with a closed pipe of the same length, the fundamental of an open pipe is —
    1. half
    2. equal
    3. double
    4. four times
    Show answer

    Answer: (C) double

    Open pipe f = v/2L and closed pipe f = v/4L, so the open pipe’s fundamental is double.

More chapters in this subject

• Mechanics36 questions
• Modern Physics36 questions
• Optics36 questions
• Properties of Matter34 questions
• Biotechnology6 questions
• Computer and Information Technology6 questions