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Question

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'. If they are connected in parallel and force constant is kk'', then k:kk':k'' is:

A

1:6

B

1:9

C

1:11

D

6:11

Step-by-Step Solution

  1. Spring Constant and Length: The force constant kk of a spring is inversely proportional to its length ll (k1lk \propto \frac{1}{l}) [NCERT Class 11, Physics Part I, Sec 6.9].
  2. Cutting the Spring: The spring of length LL is cut into ratio 1:2:31:2:3. The lengths of the segments are:
  • l1=16L    k1=6kl_1 = \frac{1}{6}L \implies k_1 = 6k
  • l2=26L=13L    k2=3kl_2 = \frac{2}{6}L = \frac{1}{3}L \implies k_2 = 3k
  • l3=36L=12L    k3=2kl_3 = \frac{3}{6}L = \frac{1}{2}L \implies k_3 = 2k
  1. Series Combination (kk'): When connected in series, the effective spring constant is given by: 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 (This is expected as reconnecting parts in series reconstructs the original spring).
  2. Parallel Combination (kk''): When connected in parallel, the effective 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
  3. Ratio: kk=k11k=1:11\frac{k'}{k''} = \frac{k}{11k} = 1:11

Exam Context & Concepts Covered

This question aligns with the NEET PHYSICS syllabus, specifically targeting concepts from LAWS OF MOTION. Mastering this topic is crucial for scoring well in the upcoming medical entrance examinations. Solving conceptually related problems will help you understand the nuances of these concepts and improve your problem-solving speed.

PHYSICSLAWS OF MOTIONspringconstantlengthsconnectedseries

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