Two balls A and B of same outer radius 2R are released from rest in water of sufficient depth. Both balls are made from same material. Ball A is completely solid and ball B is hollow upto radius R. Find ratio of terminal velocity of A and terminal velocity of B (specific gravity of balls is 2)

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TestHub · Accepted Answer

​ mg = F b ​ + F v ​ ⇒ v = 6 π P η mg − F b ​ ​ ​ v B ​ v A ​ ​ = 3 4 ​ π ( 7 R 3 ) − g − 3 4 ​ π ( 2 R ) 3 ρ g 3 4 ​ π ( 2 R ) 3 − g − 3 4 ​ π ( 2 R ) 3 ρ g ​ ​ = 7 σ − 8 P 8 σ − 8 P ​ = 14 − 8 16 − 8 ​ = 3 4 ​ ​

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