A small ring A of mass m = 
kg is attached at an end of a light string the other end of which is tied to a block B of mass 2m. The ring is free to move on a fixed smooth horizontal rod as shown. Find tension in the string when it becomes vertical.
Text Solution
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By mechancnical energy conservation
= 2 mgl
2 V b 2 + V r 2 = 4gl ....(1)
using momentum conservation mV r = 2 mV b V r = 2 V b ....(2)
2V b 2 + 4V b 2 = 4 gl 6V b 2 = 4gl V b = 
V r = 2 V b = 
when string be comes vertical velocity of block wrt to string.
V br = V b – (– V r ) = 3V b = 
T – 2 mg = 
T = 2 mg + 
= 14 mg
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