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Mechanical Properties Of Fluids

Question
CBSEENPH11019972

Two wires of diameter 0.25 cm, one made of steel and the other made of brass are loaded as shown in Fig. 9.13. The unloaded length of steel wire is 1.5 m and that of brass wire is 1.0 m. Compute the elongations of the steel and the brass wires.


Solution

Given, 
Diameter of the wires, d = 0.25 m
Hence, the radius of the wires, r = d/2  =  0.125 cm 
Length of the steel wire, L1 = 1.5 m 
Length of the brass wire, L2 = 1.0 m 
Total force exerted on the steel wire is, 
F1 = (4 + 6) g
    = 10 × 9.8
    = 98 N 
Young’s modulus for steel is given by, 
straight Y subscript 1 space equals space open parentheses straight F subscript 1 over straight A subscript 1 close parentheses open parentheses fraction numerator increment straight L subscript 1 over denominator straight L subscript 1 end fraction close parentheses

where comma space
increment straight L subscript 1 space equals space change space in space the space length space of space the space steel space wire comma
straight A subscript 1 space equals space Area space of space cross minus section space of space the space steel space wire space equals space πr subscript 1 squared space

Young ’ straight s space modulus space of space steel comma space straight Y 1 space equals space 2.0 space cross times space 10 to the power of 11 space end exponent Pa

therefore space increment straight L subscript 1 space equals space fraction numerator space straight F subscript 1 space end subscript space cross times space straight L subscript 1 over denominator left parenthesis straight A subscript 1 space end subscript space cross times space straight Y subscript 1 right parenthesis end subscript end fraction

space space space space space space space space space space space space space equals fraction numerator 98 space space cross times space 1.5 over denominator open square brackets straight pi left parenthesis 0.125 space space cross times space 10 to the power of negative 2 end exponent right parenthesis to the power of 2 space space end exponent cross times space 2 space space cross times space 10 to the power of 11 close square brackets end fraction space

space space space space space space space space space space space space space equals 1.49 space space cross times space 10 to the power of negative 4 space end exponent straight m

Total space Force space on space the space brass space wire space is comma

straight F subscript 2 space equals space 6 space cross times space 9.8 space equals space 58.8 space straight N

Young apostrophe straight s space modulus space for space brass space is comma space

straight Y subscript 2 space equals space open parentheses straight F subscript 2 over straight A subscript 2 close parentheses open parentheses fraction numerator increment straight L subscript 2 over denominator straight L subscript 2 end fraction close parentheses

where comma space

increment straight L subscript 2 space equals space space change space in space the space length space of space the space brass space wire comma

straight A subscript 2 space equals space Area space of space cross minus section space of space the space brass space wire space equals space πr subscript 2 squared

therefore space increment straight L subscript 2 space equals space fraction numerator space straight F subscript 2 space cross times space straight L subscript 2 over denominator left parenthesis straight A subscript 2 space cross times space straight Y subscript 2 right parenthesis end subscript end fraction

space space space space space space space space space space space space space space equals fraction numerator 58.8 space straight X space 1 over denominator left square bracket space left parenthesis straight pi space space cross times space left parenthesis 0.125 space space cross times space 10 to the power of negative 2 end exponent right parenthesis squared space space cross times space left parenthesis 0.91 space space cross times space 10 to the power of 11 right parenthesis space right square bracket space end fraction

space space space space space space space space space space space space space equals space 1.3 space space cross times space 10 to the power of negative 4 end exponent space straight m

So comma space

Elongation space of space the space steel space wire space equals space 1.49 space cross times space 10 to the power of – 4 end exponent space straight m
Elongation space of space the space brass space wire space equals space 1.3 space cross times space 10 to the power of – 4 space end exponent straight m.