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Question: Answered & Verified by Expert
Block A is hanging from a vertical spring and is at rest. Block B strikes the block A with velocity v and sticks to it. Then the value of v for which the spring just attains natural length is –


       
PhysicsCenter of Mass Momentum and CollisionNEET
Options:
  • A 60mg2k
  • B 6mg2k
  • C 10mg2k
  • D None of these
Solution:
1971 Upvotes Verified Answer
The correct answer is: 6mg2k
The initial extention in spring is X 0 = mg k . Just after collision of B with A the speed of combined mass is v 2 . For the spring to just attain natural length the combined mass must rise up by X 0 = mg k and comes to rest.


           


Applying conservation of energy between initial and final states

1 2 2 m v 2 2 + 1 2 k mg k 2 = 2 mg mg k

solving we get v = 6 mg 2 k

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