Q.

The length of an elastic string is x when the tension is 5N. Its length is y when the tension is 7N. What will be its length, when the tension is 9N?

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a

2y+x

b

2y-x

c

7x-5y

d

7x+5y

answer is B.

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Detailed Solution

We have, F=kx  where, F,x and k are force, length and constant respectively.  ∴  5=kx..........(1) and 7=ky..........(2)  Multiplying eq. (2) by 2 14=2ky..............(3)  Subtracting eq. (1) from (3).  14-5=2ky-kx or 9=k(2y-x)  Hence, required length =2y-x
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