Q16 / 52 · NTA Abhyas (NEET) 2020 · Physics · Mechanical Properties of Solids

Two wires A and B are of the same materials. Their lengths are in the ratio 1 :2 and diameters are in the ratio 2 :1 . When stretched by force FA and FB respectively, they get an equal increase in their lengths. Then the ratio FAFB should be

  • A
    1 :2
  • B
    1 :1
  • C
    2 :1
  • Correct answer
    D
    8 :1

Solution

Force F=YA∆ll

Substituting area A=πd22=πd24

So, we have F=Yπd2∆l4l

F∝d2∆ll ∵Yπ4=constant 

Hence, using equation (i), we have

FA∝dA2∆lAlA ...(i)

FB∝dB2∆lBlB ...(ii)

Form equations (i) and (ii), we get

∴FAFB=dA2∆lAlA×lBdB2∆lB

As ∆lA= ∆lB=∆l

Therefore FAFB= dA2dB2×lBlA

Putting dAdB=21

And lBlA=21

We get FA :FB=8 :1

Hence, the correct answer is option: 'D'.

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