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JEE Advanced 2020 Concept Recapitulation Test 4 Paper 2
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© examsnet.com
Question : 32
Total: 54
A glow-worm of mass
5.0
g
emits red-light
(
650
n
m
)
with power of
0.10
W
entirely in the backward direction. What velocity (v) in m\/sec, will it have accelerated to after 10 years, if released in free space (and assumed to live). Then
v
is?
Your Answer:
Validate
Solution:
According to the law of conservation of momentum, momentum lost by photons
= momentum gained by glow-worm Momentum lost by photons
=
no of photons
x
momentum lost by one photon
=
Totalenergy
energy of
photon
×
h
λ
[
Debroglie' equation
λ
=
h
p
⋅
p
=
h
λ
]
=
power
x
time
h
c
λ
×
h
λ
=
0.1
×
365
×
24
×
60
×
60
×
10
3
×
10
8
⇒
(
1
)
Momentum gained by glow-worm
=
5
1000
×
(
v
)
in
m
∕
s
e
c
⟶
(
2
)
(
1
)
=
(
2
)
Hence
v
=
21
msec
Ans.
v
7
=
3
.
© examsnet.com
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