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Question

A bob of mass m, suspended by a string of length l1 is given a minimum velocity required to complete a full circle in the vertical plane. At the highest point, it collides elastically with another bob of mass m suspended by a string of length l1, which is initially at rest. Both the strings are mass-less and inextensible. If the second bob, after collision acquires the minimum speed required to complete a full circle in the vertical plane, the ratio l1/l2 is

A
1
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B
3
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C
5
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D
1/5
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Solution

The correct option is C 5
Velocity of the first bob at A=5gl1
Velocity of the first bob at B=gl1
At point B is collides elastically with another bob of same mass m suspended by a string of length l1 as shown in figure.
When two bodies of equal masses undergoes an elastic collision, their velocities are interchanged.
Velocity of the second bob at B=gl1
But to complete the vertical circle, the velocity of the second bob at B=5gl2
gl1=5gl2 or l1l2=5.
867520_941508_ans_99b3faed6c9c4305b7a46241d331b495.png

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