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 -

Text Solution
Verified by ExpertsThe correct answer is:
B
The initial extension in spring is X 0 =
.
Just after collision of B with A the speed of combined mass is
.
For the spring to just attain natural length the combined mass must rise up by X 0 =
and comes to rest.

Applying conservation of energy between initial and final states
2 m
+
k
= 2mg 
solving we get v =
.
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