38.Two trains, one coming towards and another going away from an observer both at 4 m/s produce a whistle simultaneously of frequency 300 Hz. The number of beats heard by observer will be (velocity of sound = 340 m/s)(a) 5(b) 6(c) 7(d) 12 (c)M39. Speed of sound wave is v. If a reflector moves towards a stationary source emitting waves of frequency f with speed u, the frequency of reflected wave will be

(a) (b) (c) (d)

Sol.:Apparent frequency for reflector would beAfter being reflected the apparent frequency will further change and the reflector will now behave as a source. The apparent frequency will now become :

Finally we get (c)40.The intensity of sound after passing through a slab decreases by 20%. On passing through two such slabs, the intensity will decrease by (a) 50 %(b) 40 %(c) 36 %(d) 30%

Sol.:Intensity after passing through first slab = 80% of Intensity after passing through second slab = 80% of 0.8 I0 = 0.64 I0

Decrease in intensity (c)

45.A wave disturbance in a medium is described by where x and y are in meter and t is in second. Then (a) First node occurs at x = 0.15m (b)First antinode occurs at x = 0.3 m (c) The speed of interfering waves is 5.0 m/s (d)The wavelength is 0.5 m

Sol.:

Node occurs when x = 0.05 mAntinode occurs when kx = 10x = x = 0.1 m

Speed of wave (v) = = = 5 m/s

Wavelength () = = 0.2 m (c) 46.A train is moving with a constant speed along a circular track. The engine of the train emits a sound of frequency f. The frequency heard by the guard at rear end of the train (a)is less than f(b)is greater than f(c)is equal to f(d)may be greater than, less than or equal to f depending on the factors like speed of train, length of train and radius of circular track. Sol.:Let v = speed of sound and u = speed of train

Then

and f

(c)47.A massless rod AB of length L is hung from two identical wires of equal length. A block of mass m is attached at point O on the rod as shown in figure the value of AO so that a tuning fork excites the wire on the left in its fundamental tone and the wire on the right in its second harmonic is

(a) (b) (c) (d) Sol.:(d)M48.The frequency of a sonometer wire is 100 Hz. When the weights producing the tensions are completely immersed in water the frequency becomes 80 Hz and on immersing the weights in a certain liquid the frequency becomes 60 Hz. The specific gravity of the liquid is (a)1.42(b)1.77(c)1.82 (d)1.21

Sol.:

In water

or(i)

In liquid , or

From equations (i) and (ii) SL = L/w = 1.77(b)M49.A siren creates a sound level of 60 dB at a location of 500 m from the speaker. The siren is powered by a battery that delivers a total energy of 1 kJ. The efficiency of siren is 30%. The total time for which the siren sound is (a)95 s(b)95.5 s (c)96 s (d)96.5 s

Sol.:, w/m2

P = , W = 3.14 W.

(b)

52.Two waves are represented as ) and The phase difference between the two waves is

(a)(b)(c)(d)

Sol.:

phase diff. (d)55.Four sources of sound each of sound level 10 dB are sounded together, the resultant intensity level will be (log 2 = 0.3)(a) 40 dB(b) 26 dB(c) 16 dB(d) 13 dBSol.:Sound level

when four sources are sounds together, intensity

(c)56.The air in a open pipe of length 36 cm long is vibrating with 2 nodes and 2 antinodes. The temperature of the air inside the pipe is 510C. What is the wavelength of waves produced in air outside the tube where the temperature of air is 160C? (a) 32.1 cm(b) 68 cm(c) 34 cm(d) 10.2 cm

Sol.:In this mode of vibrations, Wavelength = length of the pipe = 36 cmFrequency of wave will be same inside and outside the pipe

or 34 cm (c)M65.The frequency and wavelength of the wave shown in figure are (wave speed = 320 m/s)(a)8 cm, 400 Hz(b) 80 cm, 40 Hz(c) 8 cm, 4000 Hz(d) 40 cm, 8000 Hz

Sol.:According to figure, or cm

Hz(c)

69.A transverse wave is travelling along a string from left to right. The figure below represents the shape of the string at a given instant. At this instant, among the following, choose the wrong statement(a)points D, E, F have upwards positive velocity(b) points A, B and H have downwards negative velocity(c) point C and G have zero velocity(d) points A and E have minimum velocity

Sol.:Particle velocity

or wave velocity slope of the wave

(a)For upward velocity, , so slope must be negative which is at the points D, E and F.

(b) For downward velocity,, so slope must be negative which is at the points A, B and H.(c) For zero velocity slope must be zero which is at C and G.(d) For maximum magnitude of velocity, | slope | = maximum, which is at A and E. Hence, alternative is wrong.

(d)

x = 20 cm

t

O

Y

B

C

D

E

F

G

H

X

A

Y

B

C

D

E

F

G

H

X

A

Y

L

A

B

O

m