The balloon, the light rope and the monkey shown in figure (9-E7) are at rest in the air. If the monkey reaches the top of the rope, by what distance does the balloon descend? Mass of the balloon = M, mass of the monkey = m and the length of the rope ascended by the monkey = L.
Consider origin of vertical 1-d coordinate system to be at the position of the balloon.
Initially, position of centre of mass of the system 
Finally, when the monkey reaches the top, position of centre of mass of the system = 0
Shift in centre of mass of the system 
⇒ The balloon descends by a distance of 
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