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Am elastic material has a length of 36cm when a load of 4...

Am elastic material has a length of 36cm when a load of 40N is hung on it and a length of 45cm when a load of 60N is hung on it. The original length of the string is

A.

12cm

B.

20cm

C.

18cm

D.

15cm

Correct answer is C

For Hooke's law F = Ke

F/e=Kf1/e1=f2/e2Let the original length=t0thereforee1=(36t0)cm;e2=(46t0)cmif f1=40Nf2=60NThen 4036t0=6045t040(45t0)=60(36t0)therefore 180040t0216060t060t040t0=2160180020t0=360t0=18cm