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Question

A slightly conical wire of length L and end radii r1 and r2 is stretched by two forces F and F applied parallel to the length in opposite directions and normal to the end faces. If Y denotes Young's modulus, then the extension produced is:

A
FLπr1r2Y
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B
FLYπr1r2
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C
FLπr1Y
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D
FLπr21Y
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Solution

The correct option is C FLπr1r2Y
If R be the rate of increase of radius per length, R=r2r1L
Consider a element of thickness dx at a distance x from narrow end of the wire.
The radius of the element is y=r1+Rx.
The extension in the element is dl=Fdx(πy2)Y
thus, l=FπYL0dxr1+Rx
let r1+Rx=p,Rdx=dp
or l=FπYr1+RL0dpRp2=FπYR[1r11r1+RL]=FπYR[r1+RLr1r1(r1+RL)]
or l=FLπY(r2r1).r2r1r1(r1+r2r1)=FLπr1r2Y
215573_140035_ans.png

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