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WBJEE 2020 Physics Solved Paper
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© examsnet.com
Question : 14
Total: 40
A steel and a brass wire, each of length 50 cm and cross-sectional area 0.005
cm
2
hang from a ceiling and are 15 cm apart. Lower ends of the wires are attached to a light horizontal bar. A suitable downward load is applied to the bar so that each of the wires extends in length by 0.1 cm. At what distance from the steel wire the load must be applied ?
[Young’s modulus of steel is
2
×
10
12
dynes/
cm
2
and that of brass is
1
×
10
12
dynes/
cm
2
]
7.5 cm
5 cm
10 cm
3 cm
Validate
Solution:
At equilibrium, Taking torque about point O
T
1
x
=
T
2
(
15
−
x
)
⇒
Y
1
x
=
Y
2
(
15
−
x
)
[
Y
=
T
A
×
L
l
;
T
∝
Y
]
1
×
10
12
x
=
2
×
10
12
(
15
−
x
)
;
3
x
=
15
,
So, x = 5 cm
© examsnet.com
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