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KVPY 2020-21 SB SX Exam Previous Papers
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© examsnet.com
Question : 98
Total: 120
A student is jogging on a straight path with the speed 5.4 km per hour. Perpendicular to the path is kept a pipe with its opening 8 m from the road (see figure). Diameter of the pipe is 0.45 m. At the other end of the pipe is a speaker emitting sound of 1280 Hz towards the opening of the pipes. As the student passes in front of the pipe, she hears the speaker sound for T seconds. T is in the range (take speed of sound, 320 m/s) :
6-12
12-18
3-6
18-22
Validate
Solution:
λ
=
320
1280
=
0.25
m
Using concept of diffraction of wave
b
sin
θ
=
1.22
λ
sin
θ
=
25
45
×
1.22
=
0.678
tan
θ
=
0.93
tan
θ
=
y
2
D
y
⇒
2
D
tan
θ
time to cross this region
=
2
D
tan
θ
speed
⇒
2
×
8
×
0.93
1.5
≃
9.9
sec
© examsnet.com
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