17-05-2020 JEE MAIN EXAM - PHYSICS Batch: LT21All Engg

LT21K/TP/P/[P1]/ Waves
Straight objective type(4, –1)
1. The distance between two consecutive crests in a wave train produced in a string is 5 cm. If 2 complete
waves pass through any point per second, the velocity of the wave is
1) 10 cm/sec 2) 2.5 cm/sec 3) 1 cm/sec 4) 15 cm/sec
2. The velocity of sound in a container of air at –73°C is 300 m/s. If temperature of the container were raised
to 127°C what would be the velocity of sound
300
1) 300 m/s 2) 300 2 m/s 3) 150 m/s 4) m/s
2

3. A series of ocean waves, each 5.0 m from crest to crest, are moving past the observer at a rate of 2 wave
per second. What is the velocity
1) 2.5 m/s 2) 3.8 m/s 3) 5.0 m/s 4) 10.0 m/s
4. The tension in a wire is decreased by 19%. The percentage decrease in the frequency will be
1) 19% 2) 10% 3) 0.19% 4) 13%
5. Frequency of the first harmonic in a string is 100Hz. Frequency of the third overtone is
1) 200 Hz 2) 150 Hz 3) 300 Hz 4) 400 Hz
6. The extension produced in a wire of length ‘L’ when under tension ‘T’ is ‘l’. The fundamental note emitted
by the wire when it is used in sonometer is (d = density, Y = youngs modulus of wire)

1 Yl 1 YL 1 Yl 1 Yl
1) f = 2) f = 3) f= 4) f =
2l dL 2l d 2L dL 2L d

7. If a string of mass 2.5 kg and length 20 m, under a tension of 200 N, is struck at one end. How long does
the disturbance take to reach the other end:
1) 40 s 2) 80 s 3) 160 s 4) 0.5 s
8. Compressional wave pulses are sent to the bottom of sea from a ship and echo is heard after 2 seconds. If
the bulk modulus of water is 2 x 109 N/m2 and the mean temperature is 4oC, then the depth of the sea
1) 707 m 2) 1414 m 3) 2121 m 4) 2828 m
9. Two waves of equal frequencies have their amplitude in the ratio 3:5. They are superimposed to each other.
Calculate the ratio of maximum and minimum intensities of resultant wave
1) 16:1 2) 1 : 16 3) 1 : 4 4) 4 : 1
10. The frequency of turning fork with an amplitude A = 1cm is 250 Hz. The maximum velocity of any particle
in air is equal to
3.3
1) 10 π m/s 2) 5 π m/s 3) m/s 4) 6 π m/s
π
11. Velocity of a sound wave in air:
1)Increases with pressure and temperature but decreases with humidity in air
2) Is not affected by pressure and temperature of medium but decreases with humidity
3) Decreases with temperature, increases with pressure and not affected by the humidity
4) Increases with temperature and humidity in air but not affected by pressure
LT21K/TP/P/[P1]/ 2

12. Two sound waves are represented by y1 = a sin ( wt − kx ) and y 2 = a cos ( wt − kx ) . The phase difference
between the wave is :’

1) Zero 2) π 2 3) π 4) π 4

13. Two trains move towards each other with the same speed. The speed of sound is 340 m/s. If the frequency
of the tone of the whistle of one of them heard on the other changes (9/8) times, then the speed of each train
should be :
1) 2 m/s 2) 200 m/s 3) 2000 m/s 4) 20 m/s
14. In longitudinal mode of wave propagation, the particles of the medium vibrate:
1) in the direction of propagation 2) perpendicular to the direction of propagation
3) at random 4) up and down
15. In longitudinal mode of wave propagation, the wave length is the distance between:
1) a condensation and a rarefaction 2) a condensation and the next condensation
3) a trough and next trough 4) a trough and a crest
16. A medium can carry longitudinal waves because it has the property of:
1) motion 2) mass 3) density 4) compressibility
17. As humidity increases, the velocity of sound:
1) increases 2) decreases
3) becomes zero 4) remains unchanged

18. The equation of travelling wave is Y = a sin 2π  pt −  , then the ratio of maximum particle velocity to wave
x
 5
velocity is:

πa 2πa 2πa
1) 2) 2 5πa 3) 4)
5 5 5

 t x π
19. The equation of a progressive wave is given by Y = 10cos  2π  −  +  metre. What is wavelength ?
 3 5 3
1) 10 m 2) 5 m 3) 3 m 4) 2 m
20. Beats are produced by two waves Y = a sin1000πt and Y2 = a sin 998πt , the number of beats heard / sec is
1) 0 2) 2 3) 1 4) 4
Numerical type (4, 0)
21. When an open pipe is produced third harmonic, number of nodes is
22. An ambulance blowing a siren of frequency 700 Hz is moving with a speed of 2 ms–1 towards a vertical
wall. The speed of sound is 352 ms-1. The frequency of the reflected sound as heard by the ambulance
would be:(In Hertz)
23. 56 tuning forks are so arranged in series that each fork gives 4 beats per sec with the previous one. The
frequency of the last fork is 3 times that of the first. The frequency of the first fork is:
24. A progressive wave of frequency 500Hz is travelling with a velocity 360 ms-1. How far apart are two points
600 out of phase? (in meter)
25. A wave of length 2m is superposed on its reflected wave to form a stationary wave. A node is located
x = 3cm. The adjoining antinode will be located at x equal to: (in meter)
17-05-2020 JEE MAIN EXAM - PHYSICS Batch: LT21All Engg
LT21K/TP/P/[P1]/ Waves
S 2λ
1. 1 υ= = , λ = 5cm, t = 1
T t

υ2 T2 273 + 127
= = = 2 ; i.e. υ =
2. 2
υ1 T1 273 − 73 2 2 υ1 = 300 2 m/s

3. 4 λ = 5.0 , n = 2 ; ∴υ = nλ = 10 m / s

n2
=
T2
=
81 9 (n − n )
= ; ∴ 1 2 100 = 10%
4. 2
n1 T1 100 10 n1
5. 4 3rd overtone = 4th harmonic = 4 × 100

( T / πr ) = Y ( πr 2 l ) 1 Y ( πr l )
2 2
TL 1 Yl
6. 3 Y= ; T= and n = πr d ; f =
2
=
l/L πr l
2
L 2L L(πr d) 2L Ld
2

l T m ml
7. 4 t=
v
; v=
µ
; µ=
l
;t = T1

B 2 × 10 9
8. 2 V= ;V = = 1.414 × 10 3 = 1414 ms −1 ; d = 1414 ×1 = 1414m
ρ 10 3

A1 3 T1 3
9. 1 = =
A 2 5 T2 5

10. 2 v max = Aω
11. 4
12. 2
9
[ ν = ν source and observer are moving in opposite direction ∴ apparent frequency
/
13. 4
8
v + u 9  340 + u 
ν/ = ν   ; v = v  ; u = 20 m/s
v −u 8  340 − u 
14. 1 15. 2 16. 4 17. 1
w 2πp V0 2πa
18. 3 V0 = wA = 2πpa, c = = = 5p ; ∴ =
k 2π / 5 c 5
19. 2
1000π 998π
20. 3 n1 = = 500 & n 2 = = 499, f = n1 − n 2
2π 2π
21. 3
 v + vs 
22. 708 ν′ =  υ
 v − vs 

23. 110 The frequencies are n, n + 4, n + 2 × 4,............3n .It is an A.P