Waves AC
Q.1 A tube closed at one end and containing air Q.9 The power of sound from the speaker of a radio is
produces, when excited the fundamental note of 20milli watt by turning the knob of the volume
frequency 512 Hz. If the tube is open at both ends, control the power of the sound is increased to 400
the fundamental frequency that can be excited is milli watt. The power increase in decibels as
(in Hz) compared to the original power is :
(1) 1024 (2) 512 (1) 13 db (2) 10 db
(3) 256 (4) 128 (3) 20 db (4) 800 db
Q.2 An air column in pipe, which is closed at one end Q.10 A man standing between two cliffs hears the first
will be in resonance with a vibrating tuning fork echo of a sound after 2 sec. and the second echo 3
of frequency 264 Hz if the length of the column in sec. after the initial sound. If the speed of sound
cm is be 330m/s. The distance between the two
(1) 31.25 (2) 62.50 cliffs should be:
(3) 110 (4) 125 (1) 1650 m (2) 990 m
Q.3 Velocity of sound in air is 320 m/s. A pipe closed (3) 825 m (4) 660 m
at one end has a length of 1 m neglecting end Q.11 Stationary waves are produced in 10m long
corrections, the air column in the pipe can stretched string. If the string vibrates in 5
resonance for sound of frequency. segments and wave velocity 20m/sec, the
(a) 80 Hz (b) 240 Hz frequency is-
(c) 500 Hz (d) 400 Hz (1) 10 Hz (2) 5 Hz
(1) a (2) a,b (3) (3) 4 Hz (4) 2Hz
a,b,d (4) a,d Q.12 A source x of unknown frequency produces 8
Q.4 The velocity of sound in air is 330 m/s the beats with a source of 250 Hz and 12 beats with a
fundamental frequency of an organ pipe open at source of 270 Hz. The frequency of source x is
both ends and length 0.3 metre will be: (1) 258 Hz (2) 242 Hz (3) 262 Hz (4) 282 Hz
(1) 200 Hz (2) 550 Hz Q.13 Two waves of wave length 2 m and 2.02 m
(3) 300 Hz (4) 275 Hz respectively moving with the same velocity
Q.5 A hollow metallic tube of length L and closed at superimpose to produce 2 beats per sec. The
one end produce resonance with a tuning fork of velocity of the wave is:
frequency n. The entire tube is then heated (1) 400.0 m/s (2) 402 m/s
carefully so that at equilibrium temperature its (3) 404 m/s (4) 406 m/s
length changes by . If the change in velocity V Q.14 Sound source of frequency 170 Hz is placed near
of sound is v, the resonance will now be produced a wall. A man walking from the source towards
by tuning fork of frequency. the wall finds, that there is periodic rise and fall of
(1) (V +v) / [4(L +)] (2) (V +v) / [4(L -)] sound intensity. If the speed of sound in air is
(3) (V -v) / [4(L +)] (4) (V -v) / [4(L -)] 340 m/s the distance separating the two adjacent
Q.6 When two tuning forks are sounded together x portions of minimum intensity is:
beats/sec are heard frequency of A is n. Now (1) (1/2) m (2) (3/2) m
when one prong of B is loaded with a little wax, (3) 1 m (4) 2 m
the number of beats per second decreases the Q.15 A wave of frequency 100 Hz travels along a string
frequency of fork B is : towards its fixed end when this wave travels back,
(1) n + x (2) n – x after reflection a node is formed at a distance of
(3) n – x 2 (4) n – 2x 10 cm from the fixed end. The speed of the wave
Q.7 A simple harmonic wave having amplitude A and (incident and reflected) is :
time period T is represented by the equation (1) 5 m/s (2) 10 m/s
y = 5 sin(t + 4) metre. The values of A (in (3) 20 m/s (4) 40 m/s
meter) and T(in sec.) are Q.16 The disturbance of wave propagating in positive
(1) A=5, T=2 (2) A=10, T=1
(2) A=5, T=1 (4) A=10, T=2 x-direction at t = 0 is y = and at t = 2s it
Q.8 If at a place the speed of a sound wave of
frequency 300 Hz is V, speed of another wave of becomes y = then the phase velocity
frequency 150 Hz at the same place will be:
(1) V (2) V/2 of the wave will be
(3) 2V (4) 4V (1) 1/2 m/s (2) 1/4 m/s
(3) 1/6 m/s (4) 1/8 m/s
1
Waves AC
Q.17 A uniform string of length L and mass M is fixed ( VH = 1100 m /sec), = 300 m/sec
at both ends under tension T, Then it can vibrate
(1) 11/3 (2) 11/7
with frequency given by the formula.
(3) 9/7 (4) 9/11
(1) (2) Q.25 A wave y = a sin (t – kx) on a string meets with
another wave producing a node at x = 0. Then the
equation of the unknown wave is –
(3) (4) (1) y = a sin (t + kx) (2) y = – a sin (t + kx)
(3) y = a sin (t – kx) (4) y = – a sin (t – kx)
Q.18 It the tension in a sonometer wire is increased by Q.26 A somometer wire, with a suspended mass of
a factor of four. The fundamental frequency of M = 1 kg., is in resonance with a given tuning
vibration changes by a factor of : fork. The apparatus is taken to moon where the
(1) 4 (2) (1/4) acceleration due to gravity is 1/6 that of earth. To
(3) 2 (4) (1/2) obtain resonance on the moon, the value of M
Q.19 A stretched wire of length 114 cm is divided into should be
three segment whose frequencies are in the ratio (1) 1 kg. (2) kg
1 : 3 : 4, the length of the segments must be in the (3) 6 kg (4) 36 kg
ratio : Q.27 Figure shows the shape of a part of a long wave
(1) 18 : 24 : 72 (2) 24 : 72 : 18 produced by attaching one end of string to a
(3) 24 : 18 : 72 (4) 72 : 24 : 18 tuning fork of frequency 250 Hz. What is the
Q.20 The speed of transverse waves in a stretched velocity of the waves ?
string is 700 cm/s. It the string is 2 m long, the
dispalcement
5 cm
frequency with which it resonates in fundamental 0.3 cm
0
mode is : 5 cm
0.1 cm 0.5 cm
(1) (7/2) Hz (2) (7/4) Hz
(3) (14) Hz (4) (2/7) Hz (1) 1 ms-1 (2) 1.5 ms-1
Q.21 If the velocity of sound in air is 320 m/s the (3) 2.0-1 (4) 2.5 ms-1
frequency of the fundamental note emitted by a Q.28 An observer moves towards a stationary source of
tube of length 1m closed at one end is : sound with a velocity one tenth the velocity of
(1) 80 Hz (2) 240 Hz (3) sound. The apparent increases in frequency is -
320 Hz (4) 400 Hz (1) zero (2) 5%
Q.22 An open pipe is suddenly closed with the result (3) 10% (4) 0.1%
that the second overtone of the closed pipe is Q.29 A plane progressive wave is represented by the
found to be higher in frequency by 100 Hz, than equation y = 0.25 cos (2t - x). The equation of
the first overtone of the original pipe. The a wave with double the amplitude and half
fundamental frequency of open pipe will be : frequency but travelling in the opposite direction
(1) 100 Hz (2) 300 Hz will be.
(3)150 Hz (4) 200 Hz (1) y = 0.5 cos (t - x)
Q.23 Frequency of tuning fork A is 256 Hz. It produces (2) y = 0.5 cos (t + x)
four beats/sec. with tuning fork B. When wax is (3) y = 0.25 cos (t + 2x)
applied at tuning fork B then 6 beats/sec. are (4) y = 0.5 cos (t + x)
heard. Frequency of B is : Q.30 A, B and C are three tuning forks the frequency of
(1) 252 (2) 260 Hz A is 350 Hz. A and B produce 5 beats/sec. While
(3) (1) & (2) both (4) 264 B and C produce 4 beats/sec. When A is loaded
Q.24 As shown in fig AB is one metre long cylinder. At with wax it produces 2 beats/s with B, and 6
the ends A&B and middle point C these are thin beats/s with C. The frequency of B & C is -
flexible diapharm there is H2 in AC and O2 filled (1) 341 Hz, 359 Hz (2) 345 Hz, 341 Hz
in part BC. diapharm A & B are oscillated with (3) 359 Hz, 345 Hz (4) 355 Hz, 341 Hz
some frequency when there is antinode at C then Q.31 A closed organ pipe of length 1.5 m with some
ratio of their minimum frequency na / nb will be : gas vibrate in its fundamental mode. Another
A C B
open organ pipe of same length but filled with air,
resonates with the same fork in its fundamental
H2 O2 mode if the temperature of the room be 30°C and
the speed of sound in air at 30°C be 360 m/s the
0.5 0.5
speed of sound in the gas at 0°C is:
2
Waves AC
(1) 637 m/s (2) 683 m/s m/sec., then velocity of car in m/sec will be
(3) 341.5 m/s (4) 318 m/s approximately.
Q.32 A wave y = 10 sin (ax + bt) is reflected from a (1) 8 (2) 800 (3) 7 (4) 6
dense medium at an origin. If 81% of energy is Q.39 A source of sound of frequency 1000 Hz is
reflected then the equation of reflected wave is: moving with a uniform velocity 20 m/s. The ratio
(1) y = –8.1 sin (ax – bt) of apparent frequency heard by the observer
(2) y = 8.1 sin (ax + bt) before and after the source crosses him would be:
(3) y = –9 sin (bt – ax) [Speed of sound = 340 m/s]
(4) y = 10 sin (ax – bt) (1) 9 : 8 (2) 8 : 9
Q.33 A progressive wave of frequency 500 Hz is (3) 1 : 1 (4) 9 : 10
travelling with a velocity of 360 m/sec. How far Q.40 Two sound sources (of same frequency) are
apart are two point, which have difference of 120° placed at distance of 100 meter. An observer
phase when moving between both sources hears 4 beats
(1) 0.12 m (2) 0.24 m per second. The distance between sound source is
(3) 0.18 m (4) 0.26 m now changed to 400meter then the beats/second
heard by observer will be:
(1) 2 (2) 4 (3) 8
(4) 16
Q.34 A steel wire of length 1m, mass 0.1 kg and Q.41 A sound source is moving towards a stationary
uniform cross sectional area 10–6 m2 is rigidly listener with (1/10)th of the speed of sound. The
fixed at both ends. The temperature difference of ratio of apparent to real frequency is :
wire is 20°C. If the transverse waves are set up by
plucking the string in the middle calculate (1) (2)
frequency of fundamental mode of vibration.
(Y = 2 × 1011 N/m2 , = 1.21 × 10-5 /°c)
(3) (4)
(1) 11 Hz (2) 20 Hz (3)
22 Hz (4) 15Hz Q.42 A railway engine moving with a speed of 60m/sec
Q.35 A string under a tension of 129.6 N produces passes in front of a stationary listener. The real
10 beats /sec when it is vibrated along with a frequency of whistle is 400 Hz. Calculate the
tuning fork. When the tension is the string is apparent frequency heared by listener (a) when
increased to 160N. it sounds in unison with same the engine is approaching the listener. (b) when
tuning fork. calculate fundamental frequency of the engine moving away from the listener
tuning fork. (V = 340 m/s)
(1) 100 Hz (2) 50 Hz (1) 485.7 Hz, 340 Hz (2) 220 Hz, 180 Hz
(3) 150 Hz (4) 200 Hz (3) 320 Hz,155 Hz (4) 400 Hz, 330 Hz
Q.36 For a certain organ pipe three successive Q.43 A sound source in moving with speed 5 m/s
resonable frequencies are observed at 425, 595 towards a wall. If the velocity of sound in 330 m/s the
and 765 Hz respectively taking the speed of sound stationary observer would hear beats is equal to
in air to be 340m/sec (i) whether the pipe is closed (frequency of source = 240 Hz)
end or open end (ii) determine the length of pipe. (1) 0 (2) 0 or 8
(1) closed end, 1 m (2) open end, 1 m (3) 8 (4) 0 or 4
(3) closed end, 2m (4) open end, 1 m Q.44 Doppler effect for sound depends upon the
Q.37 An under water swimmer sends a sound signal to relative motion of source and listener and it also
the surface. It is produces 5 beats/sec when depends upon that which one of these is in
compared with fundamental tone of a pipe of motion. whereas in Doppler effect for light it only
20 cm length closed at one end what is depends upon the relative motion of the source of
wavelength of sound in water. sound and observer. The reason for It is :
(take Vwater = 1500 m/sec Vair = 360m/sec) (1) Einstein's mass energy relation
(1) 3.3 m or 3.37 m (2) 4.4 m or 4.47 m (2) Einstein's theory of relativity
(3) 2.5 m or 2.7 m (4) 1m or 1.7 m (3) Photo electric effect
Q.38 A person observes a change of 2.5% in frequency (4) none of above
of sound of horn of a car. If the car is approaching Q.45 A source of sound of frequency 90 vibrations/sec
forward the person & sound velocity is 320 is approaching a stationary observer with a speed
3
Waves AC
equal to 1/10 the speed of sound. What will be the
frequency heard by the observer ?
(1) 80 vibrations/sec (2) 90 vibrations/sec
(3) 100 vibrations/sec (4) 120 vibrations/sec
