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Question: Answered & Verified by Expert
A large number of liquid drops each of radius r coalesce to from a single drop of radius R. The energy released in the process is converted into kinetic energy of the big drop so formed. The speed of the big drop is (given, surface tension of liquid T, density ρ)
PhysicsMechanical Properties of FluidsJEE Main
Options:
  • A Tρ1r-1R
  • B 2Tρ1r-1R 
  • C 4Tρ1r-1R 
  • D 6Tρ1r-1R
Solution:
2890 Upvotes Verified Answer
The correct answer is: 6Tρ1r-1R
n43πr3=43πR3

n=R3r3

ΔU=n 4πr2 T-4πR2 T

=4πT nr2-R2

=4πT R3r2r3-R2

∴   Energy released,

U=4πT R 3 [ 1 r 1 R ]

If this energy is converted into kinetic energy then

1 2 m v 2 =4π R 3 T[ 1 r 1 R ]

1 2 ×[ 4 3 π R 3 ρ ] v 2 =4π R 3 T[ 1 r 1 R ]

v= 6T ρ [ 1 r 1 R ]

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