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NEET PHYSICSMedium

A spring of force constant kk is cut into lengths of ratio 1:2:31:2:3. They are connected in series and the new force constant is kk'. Then they are connected in parallel and the force constant is kk''. Then k:kk':k'' is:

A

1:9

B

1:11

C

1:14

D

1:6

Step-by-Step Solution

The spring constant kk is inversely proportional to the length of the spring (k1Lk \propto \frac{1}{L}).

  1. Cutting the Spring: Let the original length be LL and the original spring constant be kk. The spring is cut in the ratio 1:2:31:2:3. The lengths of the segments are: L1=16L,L2=26L=L3,L3=36L=L2L_1 = \frac{1}{6}L, \quad L_2 = \frac{2}{6}L = \frac{L}{3}, \quad L_3 = \frac{3}{6}L = \frac{L}{2} Since kL=constantkL = \text{constant}, the spring constants of the segments are: k1=6k,k2=3k,k3=2kk_1 = 6k, \quad k_2 = 3k, \quad k_3 = 2k

  2. Series Connection (kk'): When connected in series, the reciprocal of the equivalent spring constant is the sum of the reciprocals of individual constants: 1k=1k1+1k2+1k3\frac{1}{k'} = \frac{1}{k_1} + \frac{1}{k_2} + \frac{1}{k_3} 1k=16k+13k+12k=1+2+36k=66k=1k\frac{1}{k'} = \frac{1}{6k} + \frac{1}{3k} + \frac{1}{2k} = \frac{1 + 2 + 3}{6k} = \frac{6}{6k} = \frac{1}{k} k=kk' = k (Note: Connecting parts back in series restores the original spring).

  3. Parallel Connection (kk''): When connected in parallel, the equivalent spring constant is the sum of individual constants: k=k1+k2+k3k'' = k_1 + k_2 + k_3 k=6k+3k+2k=11kk'' = 6k + 3k + 2k = 11k

  4. Ratio: kk=k11k=111\frac{k'}{k''} = \frac{k}{11k} = \frac{1}{11}

Thus, the ratio is 1:111:11.

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